Which of following clast sizes would be most likely transported in very fast moving water? silt cobbles sand?

I would say that in very fast moving water such as in a river, the cobble size could be transported because cobbles are 64-256mm so are quite big and in a quiescent stream won't move much but in a freshet say in a steep mountain stream they can be rolled along such as I observed in the local streams at a mining town during the Hurricane Fifi in Honduras in 1974.

The answer is d., a mountain.

A) it is created from magma