Why did the communists' use of guerrilla tactics drain american resources and cause the united states to increase the number of troops stationed in vietnam?

US did not know the geography and could not prepare for surprise attacks.

The Viet Cong and North Vietnamese troops used surprise attacks and systems of tunnels to supply the war. Peasants would also engaged in war as US soldiers were walking from place to place. Not knowing the enemy, the US increased troop numbers to combat the surprise attacks. 

The answer to your question would be b