If sewage water is added to river water, it will:

Increase the BOD of river water

The correct answer is Option (2) → Increase the BOD of river water

BOD (Biochemical Oxygen Demand) is the amount of dissolved oxygen needed by aerobic biological organisms to break down organic matter in water.

• Sewage water is rich in organic pollutants (like human waste, food residues, etc.), which increases the oxygen demand when microbes start decomposing this matter.
• When sewage is added to river water, the organic load of the river increases.
• As a result, the BOD of the river water increases, because more oxygen is now required to decompose the added organic matter.
• So: BOD of the river water increases.