Not to the "nation" (country) of the US. But the North was attacking the South...and the US was determined to stop them.
Not a direct threat like physically harming the United States, such as Japanese Submarines did to Brookings Oregon and Santa Barbara California in 1942. Or the German subs that attacked US shipping off the east coast during the war. North Vietnam was a threat, only, as a representative of the Communist powers during the cold war, N. Vietnam was acting as an agent for the USSR & Red China. North Vietnam, has sometimes, been referred to as a "Surrogate" for the USSR & Red China during the war...or someone might say, Vietnam was a "Surrogate War." In that way, yes, N. Vietnam was a threat. If the question is meant to mean, "...was N. Vietnam a threat to US forces in theater (in Vietnam)?" Yes, always. NVAF helicopters may have destroyed a US Navy Swift Boat in 1968 (PCF-19). NVAF MiG-17 jets bombed the US Navy Light Cruiser USS Oklahoma City inflicting only light damage; but heavily damaging the US destroyer USS Higbee in 1972. North Viet shore batteries constantly duelled US warships operating off of Yankee Station (North Vietnamese coastline/Dixie Station was the Southern coastline).
John Kerry, a Vietnam War veteran and later a prominent political figure, believed that the alleged threat posed by North Vietnam to the United States was overstated. During his testimony before the Senate Foreign Relations Committee in 1971, he argued that the war was based on a false premise, asserting that North Vietnam did not pose a direct threat to U.S. security. He emphasized the need for diplomatic solutions rather than military intervention, advocating for a reassessment of American foreign policy in the region.
North Vietnam was controlled by communists.
The US. Only the US was bombing North Vietnam. Although American and Australian men 'o war were bombarding North Vietnam along their coastline.
North Vietnam threatened South Vietnam. I.S.
north Vietnam took over south Vietnam
The US was an ally of South Vietnam; it was up to South Vietnam to repel the invasion from the north, with massive military assistance from the US (which, of course, ultimately failed).
South Vietnam
North Vietnam verses the US & it's allies.
Nixon.
The US wanted N. Vietnam to stop fighting in S. Vietnam.
The answer is (A) Because the North Vietnam took over the South Vietnam