The answer to this is A. a. There are a lot of people who will be confused about this since unicorn starts with the letter U which is a vowel. It should be remembered that the English grammar can be very tricky.
You cannot say that just because it starts with a vowel, you will automatically use “an.” The article “the” may be used if the word unicorn was used previously in a sentence before the given sentence. The sentence would be the continuation and “the” would be appropriate.
The word unicorn is in the middle of the sentence though. You also have to consider how you would say it. You will use a long u sound instead of a short u sound which means that the most appropriate article to use the letter a.
Context would have been helpful here. There are two articles that could be used here, depending on what comes before and after the sentence: “an” and “the”. If you’re just talking about a random unicorn, the word “an” is the right choice. This is because the word “unicorn” starts with a vowel. There needs to be some sort of a consonant sound before a vowel.
If you’re talking about a specific unicorn that was mentioned in a previous sentence (IE, this sentence belongs somewhere in the middle of a paragraph), then you want to use the word “the”. It denotes that everyone knows what unicorn you’re talking about, and that there is a specific unicorn that you want to discuss.