The correct answer is: a
The article “a” is used before singular nouns that are not specific. In this case, the doll is not specific because the speaker does not say which doll their grandmother gave them.
The article “an” is used before singular nouns that begin with a vowel sound. In this case, the word “doll” does not begin with a vowel sound, so “an” is not the correct answer.
The article “the” is used before singular nouns that are specific. In this case, the doll is not specific because the speaker does not say which doll their grandmother gave them.
The answer “none of the above” is correct because the other options are not the correct articles to use in this sentence.