To eliminate water clinging to flask walls:

Answer: C. Wash the flask with laboratory soap and rinse it thoroughly.

Do not attempt to dry the flask with a paper towel or cheesecloth. The neck is relatively narrow, and putting a paper towel or cheesecloth into it is difficult. Also, you should not add distilled water to the flask to wash the beads of water down the sides, since the solution to be diluted should be added to the flask before any water is added.