An $AR(1)$ error term looks like:

a$)lo{n}_t={}_{1}lo{n}_{t-1}+{}_{2}lo{n}_{t-2}+u_t,$ where $u_t$ is white noise.

b$)lo{n}_t=lo{n}_{t-2}+u_t,$ where $u_t$ is white noise.

c$)lo{n}_t=lo{n}_{t-1}+u_{t-1},$ where $u_t$ is white noise.

d$)lo{n}_t=lo{n}_{t-1}+u_t,$ where $u_t$ is white noise.

d

b

c