Concept:A degree is described with the preposition in when we refer to the subject or field of study.Explanation:The sentence is “You must obtain a degree … medicine to become a doctor.”The correct preposition to use with “degree” and a subject is “in”.For example, “a degree in Law” and “a degree in Engineering”.Therefore, the completed sentence is: “You must obtain a degree in medicine to become a doctor.”Answer:D. in