Answer:
Lactose intolerance.
Explanation:
Lactose intolerance is the medical condition in which the individual is unable to tolerate the milk and its related product. This occur due to the deficiency of the enzyme lactase that breaks the lactose sugar.
Clayton avoid milk as he has intestinal gas after consuming it but can take yogurt. He is suffering from the lactose intolerance as his intestine his unable to digest the lactose present in the milk. The enzyme lactose is absent or not working properly that makes his stomach upset.
Thus, the correct answer is option (b).