What will happen to an ecosystem that is experiencing a prolonged drought?

The worst case scenario is it could collapse.  If the plants cannot get the water they need to survive then they will die off.  The herbivores will have to move on to seek the vegetation even if they did not need to move on first in search of water themselves.  The carnivores would follow them.  Some of the animals might not be able to adapt to changing their environment to move to another place and they would die.  Some would be able to leave that area and move on. 

Plants will die, then the herbivores die, later the predators die and so on until the ecosystem is eradicated.

Your answer is going to be b.