What three quotes from Frankenstein chapters 21-24 show a major theme?

Throughout the book, the importance of companionship has been paramount. Victor's creature has begged for a companion, claiming that he is utterly miserable because he is alone, and he has threatened to render Victor as alone and miserable as he is if Victor refuses to comply with his demands. Even Victor, when he first isolated himself from human company to make his creature, grew desperately ill immediately upon its completion and after many months of being alone, ignoring the letters from his family, and so on. He was only able to recover his health with the support of his best friend, Henry Clerval, though it takes many months of being nursed and cared for to restore him. Companionship, it seems, is not only necessary for happiness but for survival as well.

In chapter 21, Victor is told that his friend, Henry, has been murdered, and when Victor sees the body, he says, "The human frame could no longer support the agonies that I endured, and I was carried out of the room in strong convulsions." Alone once more, Victor quickly grows ill again; with the absence of companionship, he suffers terribly and cannot get well. He convalesces for some time, but when Victor learns that his father has come to him, he says, "Nothing, at this moment, could have given me greater pleasure than the arrival of my father." No longer alone, Victor begins to feel better, and he tells Walton, "the appearance of my father was to me like that of my good angel, and I gradually recovered my health." Once again, companionship and the care of a loved one is enough to save Victor's life, showing that it is necessary for both happiness and well-being.

What quotes in Frankenstein chapters 21-24 show themes of science, bioethics, appearance, or relationships?

Frankenstein, as a whole, deals with the theme of science and bioethics with the way it explores the deep conflicts involved in Frankenstein's actions, which led to the birth of his questionable scientific creation—which he calls "monster," "fiend," and "enemy," to name a few. Its mere existence, while potentially a laboratory success for such a feat of science, carries with it the hulking weight of unethical practice, for it has assigned life to something scarcely prepared for society—and vice versa. As such, it furthers its socially unaccepted existence with socially unacceptable behavior—through murder. This, in turn, is what fills Frankenstein with the internal conflict of his own insurmountable guilt. This is evident in the following quote from chapter 21 where, even as he is being wrongly charged with murder, his trepidation lies more in his moral dilemma regarding the consequences of his creation:

Have my murderous machinations deprived you also, my dearest Henry, of life? Two I have already destroyed; other victims await their destiny: but you, Clerval, my friend, my benefactor—

Another example of the theme of bioethics can also be seen in the scene where the nurse describes her decision to uphold her moral obligation to care for Frankenstein even as she expresses her disgust for him as a man accused of murder:

(I)f you mean about the gentleman you murdered, I believe that it were better for you if you were dead, for I fancy it will go hard with you! However, that's none of my business; I am sent to nurse you, and get you well; I do my duty with a safe conscience; it were well if every body did the same.

The theme of the power of appearance and that of relationships could be found in the following chapter (22), when Frankenstein and Elizabeth profess their love for one another in the midst of such debilitating anguish. They manage to make preparations for their marriage despite Frankenstein's "emaciated frame and feverish cheeks" and Elizabeth's similar state of appearing "thinner, (having lost) much of (her) heavenly vivacity". Despite the looming miseries plaguing Frankenstein's mind regarding the threat his creature poses to his happiness and to the safety of his beloved, such is the power of appearance or beauty—as pointed out by Elizabeth—in providing much-needed, albeit temporary, respite:

Be happy, my dear Victor, ... there is, I hope, nothing to distress you; and be assured that if a lively joy is not painted in my face, my heart is contented. Something whispers to me not to depend too much on the prospect that is opened before us; but I will not listen to such a sinister voice. Observe how fast we move along, and how the clouds, which sometimes obscure and sometimes rise above the dome of Mont Blanc, render this scene of beauty still more interesting. Look ... at the innumerable fish that are swimming in the clear waters, where we can distinguish every pebble that lies at the bottom.