Solution

Stationarity is critical in time series modeling because most statistical forecasting methods assume that the series has a constant mean and variance over time. Stationary data are easier to model and predict because they lack trends and cyclical patterns that could distort analysis. The Dickey-Fuller Test, particularly its augmented version (ADF Test), is commonly used to check for stationarity by testing the null hypothesis that the series has a unit root (indicating non-stationarity). If the test rejects the null hypothesis, it indicates that the series is stationary, allowing for more reliable and robust modeling. Option A (Granger Causality Test) is incorrect as it tests causality, not stationarity. Option C (Augmented Linear Test) is incorrect because no such test exists for seasonality. Option D (Residual Variance Test) is incorrect; stationarity is not concerned with residual variance but overall series stability. Option E (Z-Test) is incorrect because it assesses differences in means, not stationarity.