Step 1: Understanding the Concept: 
The question, though slightly ill-phrased by calling physical appearance a "skill," asks where the management of one's appearance fits as a professional competency. Physical appearance is a form of non-verbal communication. 
Step 2: Detailed Explanation: 
(D) Communication skill: Physical appearance—including dress, grooming, and posture—sends powerful non-verbal messages. It influences how others perceive us (e.g., as professional, trustworthy, careless). Therefore, consciously managing one's appearance to convey a desired message is a component of the broader category of communication skills. 
(A) Counselling skill and (B) Interview skill: In both counselling and interviews, managing one's appearance is important as a part of effective communication, but it is not a standalone skill for those specific categories. It's a sub-component of the universal communication skills needed for both. 
(C) Psychological testing skill: This relates to the ability to administer and interpret psychological tests, which is unrelated to physical appearance. 
Step 3: Final Answer: 
Managing one's physical appearance is an aspect of non-verbal communication, which is a key part of overall Communication skill.