some fungai species live in plants and can kill herbivores that feed on the plant. What type of relationship does this fungus have with its host?

mutualistic

Explanation:

A mutualistic relationship is the relationship when two organisms of different species "work together," which each getting benefits from relationship.

Mutualism is conflated with the two other types of the ecological phenomena which is:

Cooperation refers to the increases in the fitness through within.

Symbiosis involves two species which are living in close proximity.

Mutualism plays a very key part in ecology. Thus, the fungi and the plant are in mutualistic relationship in which fungi is getting its food and the plant is being prevented from the herbivores which feed on them.