Answer: C. grass
Explanation:
A living organism can be define as an organism which is capable of responding to the external stimuli, growth, reproduction and development during it's life span.
Among the options given, grass is the correct option, this is because of the fact that grass describes to be a complete organism. But the egg cell, human heart are the parts of the living beings. The atmosphere is the part of earth which includes the mass of gases called as air. It is not a living organism.