Explanation:

Definition of a Pronoun:
A pronoun is a word that is used in place of a noun to avoid repetition. It refers to a noun or another pronoun in the sentence. Pronouns are used to make sentences less repetitive and more concise.

Identifying the Sentence:
The given sentence is "It is my doll." In this sentence, the word "it" is used to refer to the noun "doll." The pronoun "it" is used to avoid repeating the noun "doll" in the sentence.

Role of the Pronoun:
The pronoun "it" is used to replace the noun "doll" and act as a subject in the sentence. It helps to make the sentence grammatically correct and avoids unnecessary repetition.

Explanation of Options:
a) Noun - A noun is a word that represents a person, place, thing, or idea. In this sentence, "doll" is the noun that is being referred to by the pronoun "it." However, the question is asking for the part of speech of the word "it," not the noun it is referring to.
b) Pronoun - This is the correct answer. The word "it" is a pronoun because it is used to replace the noun "doll" in the sentence.
c) Adjective - An adjective is a word that describes or modifies a noun. In this sentence, "my" is an adjective that describes the noun "doll," but it is not the word in question.

Conclusion:
The correct answer is option B) Pronoun. The word "it" is a pronoun because it is used to replace the noun "doll" in the sentence.