The answer is (b.) Fine Pebbles have a higher threshold velocity than sand.
Given the fact that fine pebbles' diameter size are usually 2.0 to 5.0 mm which is bigger compared to sands which diameter is usually measure only at 0.05 to 2.0 mm, thus fine pebbles have higher threshold velocity than sand. In Geophysics, threshold velocity refers to the minimum velocity of a particle such as pebbles and sand to move by the cause of wind or water. Therefore, the higher threshold velocity is the fine pebble since it is bigger.
