Which of the following organisms is most likely to survive?
a. the producers in a food web
b. the top carnivore in a food chain
c. a herbivore in a food chain
d. the top carnivore in a food web

The best answer here is A. the producers in a food web.

All the other answers  are dependent on the producers, but the producers are not dependent on others: if producers die, the others will die too, but not the other way: all the others can die off, and the producers can still survive.