Explanation:

Subject Pronouns:
Subject pronouns are used to replace a noun that is the subject of a sentence. In this case, the noun being replaced is "my grandparents."

Identifying the Subject:
To determine the correct subject pronoun, we need to identify the subject of the sentence. The subject of the sentence is the person or thing that performs the action. In this case, the subject is the grandparents.

Identifying the Correct Pronoun:
The correct subject pronoun to replace "my grandparents" is "They." "They" is the third-person plural subject pronoun, which is used to refer to multiple people.

Explanation:
The sentence states that "My grandparents live in London." The word "they" is used to refer to the grandparents when they come and see us. Since "they" is a subject pronoun, it is the correct choice to replace the noun "my grandparents."

Example:
If we were to rephrase the sentence using the correct subject pronoun, it would be: "My grandparents live in London. They often come and see us."

Summary:
In summary, the correct subject pronoun to replace "my grandparents" in the given sentence is "They." This pronoun is used to refer to multiple people and indicates that the grandparents often come and see us.