What is one of the most important skills a project manager can have? A. Negotiation B. Influencing C. Communication D. Problem-solving

All the mentioned options are important but communication is one of the most important skill PM should have. Most of PM tile spend in communication only. 

Communication is the most mportant part of PM role.

A project manager should possess good communication skills if he posses that  he can negotiate he may influence people around him and problems will be solved easily.