The yellow color of the yolk is due to the presence of riboflavin. Riboflavin is a type of vitamin B2 that is found in various food sources, including egg yolk. It is a water-soluble vitamin that plays a crucial role in several metabolic processes in the body.

Option 1: Livetin is not the correct answer. It is likely a misspelling of "livetin," which does not exist. It may be confused with "ovotransferrin," a protein found in egg whites.

Option 2: Phosphate is not the correct answer. While phosphates are essential for various biological processes in the body, they do not contribute to the yellow color of the yolk.

Option 4: Vitellin is not the correct answer. Vitellin is a protein found in the eggs of some invertebrates, but it is not responsible for the yellow color of the yolk in chicken eggs.

In conclusion, the correct answer is option 3: Riboflavin, which is responsible for the yellow color of the yolk.