answer is A because if u look at the other options :
B cell division ( can not occur because there is no nucleas in the RBC, this is to increase volume available for oxygen transport )
C phagocytosis ( does not have any lysosomes, so it cannot be this option )
D protein synthesis ( as there is no nucleas, thus no DNA is present, so mRNA cannot be formed and so proteins can not be made even if there were any ribosomes )
hope you understood 
haha thanks man did the same thing

this much i had gotten easily bt now try and prove A wrong even u can do that
no mitochondria........so active transport not possible....
hmmm......i tried other conclusions.......nodin seems to fit..