The upper course of a river is the youthful stage where the gradient is steepest. This steep gradient allows for the formation of plunge pools and rapids (B).
Deltas (A), on the other hand, form at the mouth of a river in its old stage. More pronounced meanders (A) are associated with the mature and old stages of a river.
Oxbows (C) occur when the river cuts through the narrow neck of a meander via lateral erosion. This is only possible on a relatively flat floodplain in the late maturity to old stage of a river. A distributary (C) is a stream channel that carries part of a river's flow across a delta in the old stage.
Interlocking spurs (D)are definitely possible in the upper course of a river but option D is eliminated due to the presence of levees (i.e. banks of alluvial material found on both sides of a river) which occur in the mature and old stages.