The correct answer is D. none of these.
The word “best” is an adjective, not an article. It modifies the noun “policy”.
The article “a” is used before singular nouns that refer to a general class of things. For example, “I like to eat apples.” In this sentence, “apples” refers to any apple.
The article “an” is used before singular nouns that begin with a vowel sound. For example, “I like to eat an apple.” In this sentence, “apple” begins with the vowel sound “a”.
The article “the” is used before singular nouns that refer to a specific thing. For example, “I like to eat the apple that is on the table.” In this sentence, “apple” refers to a specific apple.
In the sentence “Honesty is best policy,” the word “best” is an adjective that modifies the noun “policy.” Therefore, the article “a,” “an,” or “the” is not needed.