When is the AR(1) process $x_t = \phi x_{t-1} + \varepsilon_t$ covariance stationary?

1. AFor any $\phi$, as long as the shocks have finite variance
2. BWhen $|\phi| < 1$
3. COnly when $\phi = 0$, since any persistence makes a process non-stationary
4. DWhen $\phi = 1$, which gives a stable random walk