Single-celled organisms are able to maintain internal stability because they

a) have multiple organ systems
b) work with other cells
c) contain structures that perform life functions
d) carry out photosynthesis to produce food

Eliminate B and D, not all single celled organism carry out through photosynthesis , they called singled celled organism and not work with other cell, it's either A and C, single celled organism is not a complex organism. so you can eliminate letter C.

so the best answer is A, although they have multiple organs system to maintain their internal stability.