Final answer:
An oxbow lake is formed when a meander is cut off from a stream, due to the natural processes of meandering and sediment deposition. The correct option is b.
Step-by-step explanation:
An oxbow lake is an isolated, curved lake formed when a meander is cut off from a stream. The process of meandering involves the tendency of a river to scour the outer bend of a channel and deposit sediment on the inner bend, which eventually can lead to the formation of an oxbow lake.
As a river evolves, the meanders become highly curved, and during high water events or through normal flow the neck of a meander can be breached, thus cutting off the meander and creating an oxbow lake.
Hence, Option b is correct.