What happens due to the presence of excessive nitrate in drinking water? 

Methemoglobinemia

The correct answer is option 4. Methemoglobinemia.

Excessive nitrate in drinking water can lead to a condition called methemoglobinemia, also known as "blue baby syndrome." Nitrate (NO₃⁻) is converted into nitrite (NO₂⁻) in the body, and high levels of nitrite can interfere with the oxygen-carrying capacity of hemoglobin in the blood. This interference can be particularly harmful to infants, leading to a bluish discoloration of the skin (cyanosis) and other health issues associated with oxygen deprivation. Methemoglobinemia can be serious and may require medical attention.