The question asks for elements of nutritional value found in milk besides proteins and carbohydrates.

Option 1 suggests calcium, potassium and iron. While calcium and potassium are indeed found in milk, iron is not typically a nutrient found in significant quantities in milk.

Option 2, the correct answer, provides calcium and potassium which are two key nutrients found in milk. Calcium is essential for bone health while potassium is important for maintaining blood pressure and cell functions.

Option 3 proposes potassium and iron. As earlier explained, while potassium is found in milk, iron usually is not.

Option 4 suggests calcium and iron. Again, iron content in milk is insufficient, but calcium is a significant part of milk nutrition profile.

Hence, the correct answer according to the elements of nutritional value that milk comprises is calcium and potassium.