A. The ACF(autocorrelation function) and PACF(partial autocorrelation function) can be used to visualize the correlation structure of a time series, and rapid decay to zero in both functions is an indication of stationarity. The ADF and KPSS tests can be used to statistically determine stationarity, with the ADF testing for the presence of a unit root and the KPSS testing for the absence of a unit root. To ensure accurate results, it is recommended to apply both tests, along with the ACF and PACF, to determine the stationarity of a time series.