The correct option is D Both A and C.
Reproduction is the characteristic for living organisms i.e., both unicellular and multi cellular organisms can reproduce irrespective of their cellular level. But, only multi cellular organisms have specialized organs for digestion and respiration/breathing that is GIT (Gastro Intestinal Tract) and Respiratory tract respectively, unlike unicellular organisms who respire and engulf food by general body surface. Thus, digestive tract and respiratory tract are defining feature of multicellular organism (ruminant).
So, the correct answer is 'Both A and C'.