Scientist are studying the effects of different temperatures on the processing speed of the computer ship
a. the type of computer
b. the size of the computer
c. the temperature inside the computer
d. how long l it takes the computer to perform a complex calculation.

If the scientists are studying the effects of different temperatures on the processing speed of a computer chip then they would need to know the temperature inside the computer.  

However, your questions doesn't include the full question text, so I cannot be sure what answer you need.

If the question is what do they need to know to do this, the answer is C.
If the question is how could they measure this when they subject the computer to different temperatures, then the answer is D.