which of these is an example of a biotic factor that could affect the size of a population A-climate b-disease c-a natural disaster d-water availability

Hey there!
Biotic factors are living organisms. This includes plants, animals, and bacteria. Climate, water, and natural disasters are abiotic factors, which means they are nonliving factors that affect the environment. Disease is caused by bacteria, which is a biotic factor.
Therefore, your answer is B) Disease.
Hope this helps!