Many rocky coasts occur along active margins. this is because
a.
active margins often experience repeated uplift, and thus the waves have not had sufficient time to erode the land surface down to sea level.
b.
active margins tend to have far fewer streams that feed sediment to the oceans; thus, there is not enough sand to create beaches.
c.
subduction along active margins consumes coastal sediments.
d.
active margins are associated with larger storms, which cause more erosion of sediment, leading to fewer beaches.

Many rocky coasts occur along active margins. This is because  -  

( a ) active margins often experience repeated uplift, and thus the waves have not had sufficient time to erode the land surface down to sea level.

Explanation:

Active Margin -

The Active Margin are present at the edge of the continental plate that is crashed in an oceanic plate . It is possible that the active margins are uplifted rocky coasts . Hence , the wave does not have sufficient time to erode to the surface down the sea level . therefore , many rocks are found along this active margin . As , due to regular uplifts and hence , the waves do not have sufficient time to erode the land surface down to sea level .