Explanation:

The correct answer is option 'B' - Somebody.

Understanding the sentence:
The sentence states that "Somebody has stolen my watch." This means that someone (an unknown person) has taken or removed the watch without the owner's permission or knowledge.

Explanation of the options:
Let's analyze each option to understand why option 'B' is the correct answer.

a) Some: The word "some" is used to refer to an unspecified or unknown quantity or number. However, in this context, it is incorrect because "some" is used to refer to objects or things, not people. We are looking for a pronoun that represents a person who has stolen the watch.

b) Somebody: The word "somebody" is a pronoun that refers to an unspecified or unknown person. It is the correct option because it fits the context of the sentence. It indicates that an unidentified person has stolen the watch.

c) None: The word "none" means no one or not one. However, in this context, it doesn't fit because it implies that nobody has stolen the watch, which contradicts the statement in the sentence.

d) Anybody: The word "anybody" is a pronoun used to refer to any person, whether known or unknown. Although it can be used in some contexts, it is not the correct option for this sentence. "Somebody" is a more appropriate choice because it specifically implies that an unknown person has stolen the watch.

Conclusion:
Based on the given sentence, the correct answer is option 'B' - Somebody. It accurately represents an unspecified person who has stolen the watch.