Complete the office worker's guess about the mysterious phone call.

The caller ID is blocked. It ___ be the delivery driver with our pizza, but they usually call from a local number. Let's answer it just in case.

The correct answer is might.

The speaker thinks it is possible that the caller is the delivery driver, but the word "but" introduces doubt. Because there is uncertainty, might is the perfect choice.

"Must" would mean they are absolutely sure, while "can't" would mean they think it's completely impossible.