Which question can be answered by following the scientific method? a- what was the highest temperature that was recorded yesterday? b- what is the most interesting aspect of the light-dependent reactions? c- how does a change in temperature affect the stomata of a plant? d- how many stomata are there on this leaf?

Short Answer C
D can be determined just by counting the number of Stomata. It does not require a full investigation.

B is not the answer either. You need only watch what happens during each part of the process of light hitting plants. You are not altering anything. You are  just watching.

A. Again no full experimental procedure is required. Just watch a thermometer and record the highest temperature as you see it. 

C is correct because you are setting up various temperatures and recording what happens. This is he basis of scientific investigations: start from known conditions and go to unknown ones or cause and affect.