The correct answer is c. Heterotrophs compete for the same food source.
As it can be seen from the image of the food web, the tree or plants which are autotrophs are consumed by various heterotrophs. Autotrophs provide as a source of food for several heterotrophs but several heterotrophs depend on other heterotrophs such as smaller insects or animals for their food. As shown in the food web, both frog and lizard are consumers of insects so both being heterotrophs are competing for the same source of food which is insect here.
