To keep the waste bin clean, the best option would be:
(C) Washing the bin everyday
Washing it daily will help maintain cleanliness and minimize odors and bacteria. However, if daily washing is not practical, then washing it (D) once a week is also beneficial. Keeping the bin (A) always closed can help contain odors and prevent pests, but it does not prevent dirt buildup. (B) Always keeping the bin open is not advisable as it can attract pests and contribute to odor. Overall, a combination of regular cleaning and keeping the lid closed when not in use is the best practice.