Final answer:
In order for the time complexity to be squared whenever the instance size is doubled, the growth rate must be exponential.
Step-by-step explanation:
In order for the time complexity to be squared whenever the instance size is doubled, the growth rate must be exponential. Exponential growth is characterized by a constant growth rate, where the scale increases by a factor of the base raised to the number of doubling times.
For example, if we start with t(n) = 1 and double the instance size to t(2n), the growth rate would be t(2n) = (t(n))².
Therefore, the growth rate is exponential.