Which is the best example of a simile that also creates atmosphere and mood consistent with that of The Picture of Dorian Gray? a.) "The moon hung low in the sky like a yellow skull." b.) "Time being dead, dragged a hideous future from its grave." c.) "Its very horror made him stone." d.) "Though your sins be as scarlet, yet I will make them as white as snow."
Firstly, while all four of your example quotes are trope figures of speech, only two are similes while the other two are metaphors. Similes make figurative comparisons between two unlike things by using the words, like or as and sometimes such as, e.g., Love is a big expansive thing such as a dirigible; love lies quietly in the heart as a resting rose petal; love is glorious like a red rose. A metaphor, on the other hand, compares two unlike things without the use of a comparative word, e.g., Love is a dirigible; love is a resting rose petal; love is a glorious red rose.
Secondly, the atmosphere, synonymous with mood, of the novel is best briefly depicted through the outcome of the story. Dorian is consumed by false theories of vanity and pleasure and destroys people's happiness, perverts and loses his own soul, commits murder and ends in suicide. The atmosphere of The Picture of Dorian Gray is commensurate with the dark, hedonistic, supernaturally toned doings of Dorian Gray--as he is Gray so is the atmosphere grey as in descending nightfall under a veil of dirty city fog.
Thirdly, putting it all together, the best comparative figure of speech, be it simile or metaphor, that creates an atmosphere (same as mood) consistent with The Picture of Dorian Gray seems to me to be the metaphor at (b): "Time being dead, dragged a hideous future from its grave." Notice there are no comparative words, e.g., like, as, such as, so it is a metaphor. Rationale: (1) For Dorian, time was in a very real sense dead. (2) Dorian metaphorically dragged something hideous from its metaphoric grave. (3) Dorian had what time would have revealed to be a hideous future ahead of him.