Answer is: b) Estuary
Detailed Explanation: An estuary is a coastal area where a river meets the sea
or ocean and is partially enclosed by land. When the mouth of a river appears
to be submerged, it is often due to a drowned valley caused by a rise in sea
level. In estuaries, fresh water from the river mixes with saline water from the
sea. As the river transports sediments, mud bars, marshes, and plains develop in
the estuary.
Estuaries are characterized by their dynamic mix of saltwater and freshwater,
creating a unique and productive ecosystem. They serve as important habitats
for various marine species and are also valuable for human activities. Estuaries
provide ideal sites for fisheries, as the mixing of waters creates a nutrient-rich
environment that supports diverse marine life. Additionally, the presence of deep
water in estuaries makes them suitable for ports and industries. Protection from
strong currents and tides enhances their usability for these purposes.