The correct answer is A. maintaining homeostasis. 
Homeostasis is a term referring to an organisms continuous process of maintaining and auto-regulating the conditions of its internal environment. Variables such as pH, temperature, and fluid balance need to be at optimal conditions in order for the organism to function properly.
In this example, the phosphate buffer system permits the organisms to maintain a constant pH in their intracellular fluid. This is one of the organism's homeostatic mechanisms.      
        
             
        
        
        
Answer:
Vestigial organs are a type of homologous organs.
Explanation:
Vestigial organs are rudimentary in present organisms. Vestigial organs such as wisdom teeth,nictating membrane of eyes,body hairs,which were functional organs in ancestors. Though they are present in an organism in reduced form but are functionless.
Examples of vestigial organs:
 1.  Human tail or appendix,
2. Pelvic bone of a snake
3. Wings of flightless birds
4.Male breast tissues and nipples