Which word in the sentence does the adverb clause modify?

The day feels warmer since the sun came out.

a. since

b. warmer

c. feels

d. day

The adverb clause here is "since the sun came out" - it gives some information about the timing of the event.

It modifies "feels" (actually I would say that it modifies the phase "feels warmer") - this "feeling" happens since the sun came out, that's when it started!