Nato was formed to a) encourage allies to supply military goods to the united states. b) ensure mutual protection from communism. c) bolster the global us military presence. d) heighten security for allied nations.

NATO was formed to B. ensure mutual protection from Communism.
NATO was created when the Soviet Union grew too strong - it was threatening to spread communism all over Europe and farther, which nobody really liked. This is why several states allied with America in order to combat the spread of such ideologies.