Many wild orchids cannot be successfully dug up and transplanted because they need the native mychorrhizae (a fungus) in the soil to survive. what kind of interaction is this?

The correct answer is mutualism.  

Mutualism also knew as interspecific cooperation is the manner in which two organisms of distinct species prevail in an association in which each individual is benefited from the action of the other.  

In the given association, the plant gives energy and protection to the fungus and the fungus in return offers nutrients in the soil to the plant.