A-Food molecules ; photosynthesis produces food molecules i.e. glucose.

B¾ Photosynthesis; Co2 is used in photosynthesis.

C¾ Respiration; Respiration releases CO2 so C is respiration

(b) The given cycle is oxygen cycle because atmospheric oxygen is taken up in C respiration to give out CO2 which is used in B photosynthesis to produce food i.e. A