As stated above, the answer to your question is most likely all of the above. The first, long part of the book on cetology (whaling) can discourage even the most stout hearted from continuing to read the book. Melville felt it was necessary to set the scene for his book, but imagine if this book were to be published today; I suspect virtually any editor hoping for decent sales would cut it down to about a page. As for the rest, Ishmael (if that is truly his name) is the least developed character while many of the others are well developed. Obviously Ahab is a character whose depth is studies in the story, but so are many even minor characters such as the minister in the whaler's church and the owners of the ship. You want symbols? The book is full of them, from the opening sentence of the main text to the last. Finally, there is a romantic adventure tale here as well. Melville managed to create a great work of art that can be many things to many readers. Perhaps that is why his compendium had to be so long. I was once told by a professor that Stephen Crane, author of the famously brief novel "Red Badge of Courage," read "Moby Dick" and wrote in the margin of the text, "Great book, too damn long." Perhaps.
The most appropriate answer is (e) all of these.
The critic Tony Tanner, in his introduction to the 1998 Oxford World's Classic edition of Melville's seminal work, contends that this novel could only have been written in mid-nineteenth century America, a "time of dominion and expansionary confidence in the western world." For, Tanner continues, Melville generates both an epic and a myth in his work with the many chapters that objectively categorize whales and dissect the business of whaling also directing the reader symbolically to the mythical majesty of the largest creature of the sea. Walking the decks of the whaling ship that seeks the inscrutable white whale, Moby Dick, is Captain Ahab, monomanical in his desire for revenge against the creature, who speaks in an elevated style suitable to the epic hero/anti-hero. In this elevated style, also, Ahab examines the metaphysical characteristics of the great whale, who is but "a pasteboard mask" that hides its inscrutable malice; this pasteboard mask Ahab swears he will break through:
That inscrutable thing is chiefly what I hate; and be the White Whale agent, or be the White Whale principal, I will wreak that hate upon him. Talk not to me of blasphemy, man; I'd strike the sun if it insulted me.
Ahab vows to find the truth, truth that has "no confines." And, he sets out upon a voyage that ends in more than death.