What choice fits best in: Sorry, I _____ go to the bank at lunchtime?

The sentence "Sorry, I _____ go to the bank at lunchtime" suggests an obligation or necessity to go to the bank. The word "must" indicates a strong obligation or requirement, which fits well in this context. Using "must" implies that there is a compelling reason for needing to go to the bank, such as an urgent requirement that cannot be postponed.

Other options do not convey the same level of necessity. "Ought to" and "should" imply recommendations or suggestions rather than definitive obligations. They express a sense of advisability rather than urgency. "Have to" also indicates obligation, but it is often used in more informal contexts or in situations where the necessity is slightly less urgent than what "must" conveys. Therefore, "must" is the most fitting choice for emphasizing the obligation in this context.