Which condition at a river mouth is most decisive for delta formation?

Correct answer: Deposition of sediments when velocity decreases

A delta develops where a river enters a sea, lake, or another low-energy water body and loses speed. When the current becomes slower, it can no longer carry all the sand, silt, and clay that it transported downstream. These sediments settle near the river mouth and gradually build outward, often forming several distributary channels. Thus, deposition caused by reduced velocity is the decisive process.

Option A gives both necessary ideas: the river slows down and its sediment is deposited. A gorge is generally associated with strong erosion in a steep, confined valley, not with delta building. A river does not become a glacier at its mouth, and disappearance in a desert is an inland drainage phenomenon rather than delta formation. Therefore, option A follows directly from the process.