Which of the following are not abiotic factors of an ecosystem?
a. water abundance
b. temperature
c. day length
d. herbivores

The correct answer is letter D. Herbivores are not an abiotic factor in an ecosystem because these factors refer to chemical and physical parts of the environment. The general location where organisms thrive and how they function in the ecosystem is affected by abiotic factors, letters A, B, and C refer to those while D is not. 

The factors which aren't abiotic from the ones you've provided us with here would be D - HERBIVORES.

A general rule of thumb would be that abiotic factors are factors which are non-living while living facors would be considered as biotic factors.