Marine water is rich in salt concentrations and they render a salt taste to the sea water. As an estuary is formed by both the marine and fresh water, it cannot be salt water only. Hence, option (a) is incorrect.
When river meets the sea, it brings with it huge amount of sediment and organic matter. Generally, the upper layer of the soil which is rich in nutrients gets washed away by the running water. Often, when there are floods, soil erosion occurs rapidly. Therefore, the brackish waters are generally rich in nutrients. Hence, option (b) is incorrect.
The concentration of salts in the fresh water is less and thus, the fresh water from the river seems to be tasteless. As an estuary contains a mixture of fresh water and sea water, it cannot be fresh water only. Hence, option (c) is incorrect.
Estuaries are generally formed in the shallow regions and hence good amount of sunlight penetrates into the water. Such estuarine regions are often included in the photic zones and not in the aphotic zones. Hence, option (d) is incorrect.
An estuary is formed as a result of mixing up of the sea water with the river water. This occurs at the junction where the river opens in to the sea. Therefore, the water here is a mixture of fresh water and salt water and is called the brackish water. Hence, option (e) is correct.
Therefore, the water in an estuary is (e) a mixture of fresh water and salt water.