The correct answer is D. no article.
The sentence “It is going to rain. I must buy an umbrella quickly” is grammatically correct, but it is not idiomatic. In English, we do not use articles with general nouns, such as “umbrella.” We would say “I must buy an umbrella” if we were talking about a specific umbrella, such as “I must buy the umbrella that I left at home.” However, if we are talking about any umbrella, we would say “I must buy an umbrella.”
In this case, the speaker is not talking about a specific umbrella, but rather any umbrella. They are simply saying that they need to buy an umbrella because it is going to rain. Therefore, the correct answer is D. no article.
Here are some other examples of sentences where we do not use articles with general nouns:
- I need to buy a car.
- I want to get a job.
- I need to find a place to live.
In each of these sentences, the speaker is not talking about a specific car, job, or place to live. They are simply saying that they need to find one.