To determine the correct form to complete the sentence "When you come to London you ...... come and see us," we need to analyze the meanings of the options given: "mustn’t," "has to," "must," and "have to." Let's break down each option:

• Mustn’t: This is a contraction of "must not," which indicates prohibition or something that is not allowed. For example, "You mustn’t smoke here." In the context of our sentence, this would imply that you are not allowed to come and see us, which does not fit the intended meaning.
• Has to: This phrase indicates obligation or necessity. It is often used in the present tense. For example, "She has to finish her homework." While this could imply that visiting is necessary, it doesn’t sound as natural in this specific context.
• Must: This word expresses a strong obligation or necessity. It is used to indicate that something is very important or required. For example, "You must try the local food." In our sentence, this implies that it is very important for the person to come and see us when they are in London.
• Have to: Similar to "must," this phrase also indicates obligation but is often used in a more general context. For example, "I have to go to work." It can also fit in our sentence, but "must" carries a stronger sense of urgency or importance.

Considering the context of the sentence, where the speaker is likely expressing a desire for the person to visit, the best option is must. The completed sentence would read:

When you come to London you must come and see us.

This conveys a strong suggestion or invitation to visit, which aligns perfectly with the intent of the speaker.