What was the Tet Offensive?

an all-out invasion of North Vietnam by South Vietnamese and American troops

a combined North Vietnamese and Chinese attack on the American navy in the Gulf of Tonkin

an American bombing campaign against Communist forces in Laos and Cambodia

a series of surprise attacks launched by North Vietnamese forces against every major
city in South Vietnam

This is the right option:

A series of surprise attacks launched by North Vietnamese forces against every major city in South Vietnam.

Explanation:

Tet is the Vietnamese New  Lunar Year. Depending on the calendar, it starts either in January or in February. On New Year´s Day 1968, the North Vietnamese army and the Viet Cong guerrillas launched a surprise attack on South Vietnamese and American troops in many cities along South Vietnam, including Saigon and Hue.

After a few weeks, the communist forces were pushed back from all cities. They retreated with no significant military victories, but the Tet Offensive was a major political and PR sucess for the Vietnamese communists; it showed the chaos and uncertainty of the war and gave the US public the impression  that war was far from being won as the US government wanted the public to believe. It strengthened the pacifist movement in the USA.

b. introduce the core ideas of the new government.