The correct answer is D. None of the above.
The word “poor” is an adjective, not a noun. It describes a person or group of people who are lacking in money or material possessions. Therefore, it does not take an article (a, an, or the).
The article “a” is used before singular nouns that refer to a general or unspecified person or thing. For example, “I saw a man walking down the street.”
The article “an” is used before singular nouns that begin with a vowel sound. For example, “I saw an elephant in the zoo.”
The article “the” is used before singular nouns that refer to a specific person or thing that has already been mentioned or is known to the listener. For example, “I saw the man who stole my car.”
In the sentence “We must help the poor,” the word “poor” is an adjective that describes the noun “people.” Therefore, the article “the” is not necessary.