Answer: D. It is a verb that means "to come between"
Explanation: The word is clearly a verb, so the two first options are discarded. The word "intervene" is composed of "inter-" (Latin prefix meaning literally "between") and "vene" (from <em>venire,</em> which means "to come" in Latin). When a subject intervenes, it is generally implied that it comes between two or more subjects engaged in a conflict.