Question

how often should people wash their hands to maintain good personal hygiene?
once per day
at least twice per week
whenever their hands are visibly dirty
whenever they touch unclean surfaces

Explanation:

Maintaining good personal hygiene involves washing hands when visibly dirty as it directly removes dirt and potential pathogens. Washing only once a day or twice a week is insufficient. While touching unclean surfaces is a reason to wash hands, it is a more specific trigger compared to visible dirtiness which is a clear - cut sign for hand - washing.

Answer:

C. whenever their hands are visibly dirty