Which of the following assumptions is required to obtain a first-differenced estimator in a two-period panel data analysis?

The assumption required to obtain a first-differenced estimator in a two-period panel data analysis is that the model is stationary or its components have a unit root. This means that the variables in the model do not grow or decline over time and thus the differences between the two-periods are the only meaningful measure of change. This helps to control for factors that may be related to both periods, as well as ensuring that the difference between the two periods is the only measure of change.