What event was the direct cause of the cuban missile crisis?

Placement of MRBM on Cuban soil

Explanation:

Cuban Missile Crisis was triggered by the Soviet placement of MRBM (Medium Range Ballistic Missile) on Cuban soil, capable of reaching the majority of the continental United States.

This placement of MRBM would render most, if not all US anti-missile system useless, due to its closeness to the mainland US (thus smaller time frame to react).

This placement itself was a countermove by the Soviets. Before, the US had placed its own MRBM on Italy and Turkey, rendering the Soviets' own anti-missile system useless.

In the end, the aftermath of the Cuban Missile Crisis saw the Soviets and the US moving back their MRBMs.

i beleive a. is the only possible answer