He is proficient _____ both code and design.

He is proficient _____ both code and design.

• A. At
• B. On
• C. With
• D. In

Answer: (D)

Using the expression proficient in signals the area of expertise. When you want to say someone has strong skills in a field, you pair proficient with in and then name that field. So the natural completion is proficient in both code and design—the preposition in links to the domain of skill, indicating competence within those areas. At, with, and on are not as natural here for expressing domain expertise. At tends to go with general abilities or tasks (good at coding), with points to the tool or resource you’re using (proficient with Java, for example), and on isn’t used to denote mastery of fields in this context.

Using the expression proficient in signals the area of expertise. When you want to say someone has strong skills in a field, you pair proficient with in and then name that field. So the natural completion is proficient in both code and design—the preposition in links to the domain of skill, indicating competence within those areas.

At, with, and on are not as natural here for expressing domain expertise. At tends to go with general abilities or tasks (good at coding), with points to the tool or resource you’re using (proficient with Java, for example), and on isn’t used to denote mastery of fields in this context.