He was suffering from boredom, the security guard would let him grind his favorite spot on his what?\nOptions:\n- meet interesting people\n- listen to music\n- deck\n- play cards\n- skateboard
A step-by-step solution is:
skateboard
Grinding is an activity that is performed using a skateboard.

Skateboard is a thing.

Grinding on a skateboard involves sliding on it.

.

Final answer: skateboard