Question

Which of the following is a density-independent limiting factor?

Answer 1

Most density independent limiting factors are abiotic (nonliving). So, examples could be temperature, flood, pollution, earthquakes, etc. In your case, the answer would be C. The eruption of a volcano. 
Answer A is wrong because that is a description of competition between organisms, which is a biotic factor. 
Answer B is wrong because those are both biotic and density dependent factors.
Answer D is wrong because disease is potentially biotic, but primarily because parasitism is mentioned, which is interaction between two organisms. One benefitting, and one being "hurt" in some way.