What relatively successful tactic did North Vietnamese and Vietcong fighters employ against U. S. forces?

The Vietcong built thousands of kilometres of tunnels and complex underground shelters to avoid US air raids and reduce casualties. Often US troops were killed by traps. The Vietcong had much support in the villages, and those who did not support them were terrorised into provided shelter and food.

i think your answer there is c

d) a constitutional amendment