The correct answer is option 2, Nitrate. Blue baby syndrome, also known as methemoglobinemia, is a condition that affects infants and is characterized by the bluish tint of the skin. Nitrate is responsible for this syndrome because it can be converted into nitrite, which can then react with hemoglobin to form methemoglobin. Methemoglobin is unable to bind and transport oxygen efficiently, leading to a decreased oxygen supply in the body. This lack of oxygen manifests as a bluish coloration in the skin.

It`s important to note that while the other options listed (Fluoride, Arsenic, and Lead) can have harmful effects on the body, they are not specifically known to cause blue baby syndrome. Drinking water contaminated with high levels of fluoride can lead to dental fluorosis, arsenic can cause various health problems including skin lesions and cancer, and lead exposure can harm the nervous system. However, none of these substances directly cause blue baby syndrome.