Which of the following is not a manmade cause of global warming?
a. burning fossil fuels
b. deforestation
c. respiration
d. population growth

Burning fossil fuels surely is a manmade cause of global warming and so is deforestation, we are the ones who are cutting down huge amounts of forest to get the required wood and make clear the way for highways. 

We are also responsible for population growth - we are the ones who're procreating in the end. However, population growth by itself is not always put into the category of causes of global warming. Another possible thing is respiration in this case.

So to finally answer completely - the best answer in this case is respiration as the fact that respiration is happening least affects global warming among the options listed.