Explanation:

The correct answer is option D - none.

Reasoning:

In this sentence, the speaker is referring to a specific location where their sister lives. The correct article to use before London depends on whether London is being used as a proper noun or a common noun.

- If London is being used as a proper noun, we do not use an article before it. Proper nouns are specific names of people, places, or things, such as names of cities, countries, or individuals. In this case, "London" is a proper noun because it is referring to the specific city rather than a general city. Therefore, we do not need to use any article before London.

- If London is being used as a common noun, we need to use an article before it. Common nouns are general names of people, places, or things. In this case, "London" would be a common noun if the speaker is referring to any city called London. In this case, we would need to use the definite article "the" before London.

Since the sentence does not provide any additional context, we cannot determine whether London is being used as a proper noun or a common noun. Therefore, we should not use any article before London.

Example:

- My sister lives in London. (Correct)

This sentence does not use any article before London because it is referring to a specific location (proper noun). The speaker is talking about a specific city called London.

Conclusion:

In the given sentence, the correct option is D - none. No article is needed before London because we cannot determine whether it is being used as a proper noun or a common noun.