Answer: bacteria
Step-by-step explanation:
A biotic factor refers to the living component of an ecosystem. That is, any living organisms present.
Hence, amidst temperature, sunlight, bacteria, or water, the only biotic (living) factor is bacteria, since only bacteria can feed, move, excrete, reproduce and show all other characteristics of living organisms.
On the other hand, temperature, sunlight, or water all represent abiotic (non-living) factors.