The blocking off of an estuary from the sea forms a(n)

The correct answer is - lagoon.

When rivers and streams that have lower water volume throughout the whole year come in touch with the ocean, under the influence of the ocean's wave and ocean currents along the coast, sandbars and barrier islands start to form around the estuary. As they form and little by little grow in size, they manage to create a closed space with calm shallow water between the estuary itself and the ocean, thus creating a lagoon.