Which is a biotic factor that affects population size? a temperature b food c precipitation d available sunlight

Biotic: " Produced or caused by living organisms" (definition excerpted from link below)  

Since the temp., sunlight and precipitation are not produced or caused by living organisms, they cannot be the answer, since they are abiotic.  

Since food is created by living organisms, food must be the answer. As a matter of fact, food is a huge factor in the growth of a population of anything, from bacteria to baleen whales and certainly humans.

c.more precipitation,but only when emission levels.

if your own plato the answer is b. the fern transitioned from sporophyte generation to gametophyte generation.