What would most likely happen to the food web if the amount of grass drastically decreased?

All of the consumers would be affected, and be forced to adapt.  Since the amount of grass has decreased, the primary consumer would struggle to survive, and most likely, die out.  Since the primary consumer has probably died out, the secondary would either die, or be forced to adapt to the situation, or change habitats ( applies to the rest in the food chain / web).

The correct answer is  b) 

The answer is overmatching