Secondary productivity is

Secondary productivity can be defined as the rate by which biomass formation occurs or energy is fixed by heterotrophic organisms in an ecosystem like grazers and decomposers.

These organisms obtain their energy from photosynthetic plants and other autotrophic organisms present at certain trophic levels.

The productivity of these organisms determine the number of trophic levels and the length/size of food chains within an ecosystem.

The rate by which the consumer consumes food and converts the chemical energy of their food into their own biomass is called secondary productivity.

Hence the correct answer is C that is the rate of energy storage at consumer level.