Eutrophication causes reduction in:
A) Nutrients
B) Dissolved salts
C) Dissolved oxygen
a) A
b) B & C
c) C
d) A, B &C

Hint: Eutrophication occurs when there is an overgrowth of harmful algae and overloading of water with nutrients. Thus, the water becomes polluted and unable to sustain any form of aquatic life.

Complete step by step answer:
Let us first see how eutrophication occurs:
Natural water bodies can tend to form brackish water after long periods of time. This is due to the accumulation of water-soluble nutrients in excess amounts. What eventually happens is that these nutrients provide a suitable growth for algae and other microorganisms. This phenomenon is known as an algal bloom.
As we know, algae tend to use up a lot of dissolved oxygen for their growth. In addition to this, the presence of organic matter will also decrease the level of dissolved oxygen as they are broken down by bacteria. Thus, as we can see, this leads to the water body being unfit for the sustenance of aquatic life.
Hence, in eutrophication, as we can see from our above observations, the level of nutrients and dissolved solids go up, while there is a reduction in the level of dissolved oxygen.

So, the correct answer is Option (c).

Note: Eutrophication causes reduction in dissolved oxygen .It is mainly produced due to human activities (also known as anthropogenic activities), and this case is known as accelerated eutrophication. The main reasons for this are uncontrolled release of pesticides and fertilizers which seep into water bodies, and the dumping of waste into water bodies. Release of untreated wastewater effluents from factories is also a major cause of eutrophication.