A chemical messenger is any compound that serves to transmit amessage. A chemical messenger may refer to: Hormone, Long rangechemical messenger. Neurotransmitter, communicates to adjacent cells.
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.
