The correct answer is C. Proficient.
Adept means having a high level of skill or expertise in a particular area. Proficient means having the necessary skills and knowledge to do something well. Intelligent means having a high level of intelligence or mental ability. Sufficient means enough or as much as is needed. Professional means relating to or belonging to a profession.
Adept is the most similar to proficient because both words refer to having a high level of skill or expertise in a particular area. Intelligent is not a synonym of adept because it refers to having a high level of intelligence or mental ability, while adept does not necessarily refer to intelligence. Sufficient is not a synonym of adept because it refers to being enough or as much as is needed, while adept does not necessarily refer to being enough or as much as is needed. Professional is not a synonym of adept because it refers to relating to or belonging to a profession, while adept does not necessarily refer to a profession.