Step 1: Define "professional services". Professional services are occupations in the tertiary sector that require specialized knowledge, training, and often formal qualifications or licensing. They are knowledge-intensive services.
Step 2: Analyze the options.
An Advocate (lawyer) requires extensive education (a law degree) and a license to practice. This is a classic professional service.
A Washerman and a Barber provide valuable personal services, but these occupations are typically classified as skilled or semi-skilled labor rather than professional services, as they do not usually require the same level of formal academic training and certification.
Step 3: Conclude based on the definition. An advocate is the clearest example of a professional service among the choices.