Which of the following is a density-independent limiting factor?
a. a fm struggle for food, water, space, or sunlight
b. predator/prey relationships
c. the eruption of a volcano
d. parasitism and disease

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.

From a high concentration to a low concentration.