The correct answer is option 3, protein. Nitrogenous food refers to any type of food that contains nitrogen, and proteins are known to be the primary source of dietary nitrogen. Proteins are macromolecules made up of amino acids, which contain nitrogen atoms. Therefore, when we consume protein-rich foods like meat, fish, eggs, dairy products, legumes, and nuts, we are consuming nitrogenous food. It`s important to note that while carbohydrates, lipids, and salts are also essential components of our diet, they do not necessarily contain nitrogen. Carbohydrates are composed of carbon, hydrogen, and oxygen, lipids are composed of carbon, hydrogen, and oxygen but do not contain nitrogen, and salts primarily consist of ions, not nitrogen-containing molecules. Therefore, the correct answer is protein as it is the only option associated with nitrogenous food.