You are a procurement manager and are responsible for managing complex procurement contracts.

I say B - Formal written communication............Thanks

formal written is a documented method , so it is easy to refer to it if there is any conflict

I apologize for the answer I leave the answer to the experts, specialists in this field that's not my specialty field.

Thanks

It is Formal Written ........................

- verbal communication.........................

The most effective means is :

Formal written

Since the situation mentioned it is complex contract and it is in relation with a contractor the best choice is formal written