The sentence refers to a past action at a specific point in time, which requires the past simple tense.
In negative sentences using the past simple, the auxiliary verb "did" is used, followed by the base form of the main verb.
The correct sentence should be:
"He didn't know the answer to the question."
Thus, the correct answer is (B) "didn't know."
Each question has a sentence that has an underlined part. Choose the option which rephrases the underlined part in the most appropriate manner in terms of grammar and usage.
In an attempt to boost their profits, many edible oil-producing companies have been engaging themselves in propaganda against commonly used oils and promoting exotic and expensive varieties of oils \(\underline{as\ more\ healthier\ options.} \)