The correct answer is C) it halted the infiltration of the Communist in South Vietnam.
As a result of US advisors, the United States bombed part of North Vietnam. The effect this had on North Vietnam was that it halted the infiltration of the Communist in South Vietnam.
The United States Air Force bombed North Vietnam trying to accomplish the following things. First, to halt the infiltration of the Communist in South Vietnam. Secondly, to boost the morale of the South Vietnamese army, and finally, to demand Hanoi to stop its support to the rebels against South Vietnam.
The other options of the question are A) it instilled fear, B) a quick settlement was offered by North Vietnam but the United States refused., and D) it elevated the morale of the North Vietnamese.