C) The stairs, because rhythm is repeated elements in photography, like books/kayaks/chairs in a row.
A) would be emphasis, not rhythm, because there's no repetition of the blue light (it's emphasis because the spiral staircase leads to the blue light).
B) would be line, not rhythm, because there are no repeated elements.
D) would be perspective, not rhythm, because being high up doesn't have anything to do with rhythm.
