What best explains why there's fewer secondary consumers than producers?

because of inefficiency

Explanation:

The organisms that eat the primary consumers are meat eaters (carnivores) and are called the secondary consumers. ... Because of this inefficiency, there is only enough food for a few top level consumers, but there is lots of food for herbivores lower down on the food chain. There are fewer consumers than producers.

complete the square on the right side of the equation;

-5(x + 0)^2 + 15

answer: d. y = -x^2 + 15