None of us(have/has)a good opinion of Sally.

Can u explain the answer too?

None of us has

None is the subject of the verb. It is considered a contraction of 'not one' and is singular.

'of us' is known as a prepositional phrase. It plays no role in determining the verb.

For example:

A box of tapes is in the car.

'of tapes' plays no role.
'box' is the subject (hence 'is')