Energy enters in a food chain through:
(a) primary consumers
(b) secondary consumers
(c) tertiary consumers
(d) producers

(d) producers

Reason — In a food chain, the first level are the producers (photosynthetic plants). They utilize the energy received from the sun to synthesize organic substances by the process of photosynthesis (i.e., they combine simple compounds with the help of solar energy into complex organic substances).