The correct answer is option 4: Casein. Casein is the main protein found in milk. It accounts for about 80% of the total protein content in milk. Casein is a slow-digesting protein that provides a sustained release of amino acids, making it an excellent protein source for muscle recovery and growth.

Now let`s look at the other options to understand why they are not the correct answers:

Option 1: Albumin is a protein found in eggs and blood, but it is not the main protein in milk.

Option 2: Globulin is a type of protein found in blood plasma, but it is not the main protein in milk.

Option 3: Globin is a component of hemoglobin, the protein responsible for transporting oxygen in the blood. It is not found in milk.

Therefore, the correct answer is option 4: Casein.