D.
Option D indicates producers that can make their own food, they provide most available energy but they use a part of energy for their own purpose and the remaining energy is gained by the next group lying above them in the pyramid so as we go up the energy declines.
Mathematically,
Suppose E1 is the energy of the bottom group
a is the energy they use for their own purpose
b is the remaining energy
So,
remaining energy = total energy - used energy
i.e. b = E1 - a
Now, b is the total energy for the next group so as we go up the pyramid, the energy becomes lesser and lesser.