The correct answer is (B) To weaken (in force or effect); to water down (doctrine, zeal, story, etc.); to reduce strength of (fluid); to diminish brilliance of (color); to make less concentrated by adding more water or a thinner, etc.

Dilute comes from Latin diluere (to wash away).

Examples of use:

• He added water to the reagent to dilute its strength.
• Their argument has been diluted by the discovery of the missing papers.