Final answer:
Gravel requires the highest current velocity to be transported due to its larger and heavier particles compared to sand, silt, and clay.
Step-by-step explanation:
The sediment particle size that requires the highest current velocity to be transported is gravel. When we consider the different sizes of sediment particles such as gravel, sand, silt, and clay, gravel particles are the largest and heaviest, thus needing more energy to be moved by water. In clastic sedimentary environments, larger particles settle first when the transport energy decreases because they are too heavy to be carried by slower currents. Gravel has larger particles than sand, silt, and clay, so it will settle out sooner in a depositional environment, which means there must have been a higher current velocity to move it initially. Graded bedding in sedimentary layers also indicates the role of current velocity in transporting sediment; the bottom layers consisting of gravel imply a higher energy deposition compared to the finer layers above. Hence, the highest current velocity is needed to transport gravel.