Erosion can primarily lead to a lower beach elevation. When waves and currents wash away sand from the beach, the overall height of the beach decreases, resulting in a lower elevation. This can also lead to narrower beaches in some cases, as the sand is removed and not replaced.
The other options, like wider beaches, increased sand on beaches, and higher beach elevations, typically do not occur due to erosion; rather, those conditions might be associated with accretion, which is the process of sediment being added to the beach.
In summary, the most direct effect of erosion on a beach is lowering its elevation.