To determine if a number is a happy number by repeatedly replacing it with the sum of the squares of its digits until it reaches 1 or falls into a cycle (which would indicate it is unhappy), you would typically use a while loop.
This is because you want to keep iterating over the process of computing the sum of the squares of the digits until you reach a termination condition (either reaching 1 or detecting that you've entered a cycle).
Therefore, the correct choice would be:
while loop