= Solution
At lag $h$, the plot shows the <sample autocorrelation function>
$$
\widehat\rho(h)=
\frac{\sum_{t=h+1}^n(X_t-\overline X)(X_{t-h}-\overline X)}
{\sum_{t=1}^n(X_t-\overline X)^2}.
$$
Under a <white noise process>, each fixed nonzero-lag sample autocorrelation is approximately $N(0,1/n)$, so the dashed pointwise $95\%$ reference lines are approximately $\pm1.96/\sqrt n$.
The first nonzero-lag bar is well above the upper line, which contradicts the zero autocorrelation expected from white noise. Since the plot then largely cuts off, an <moving-average process of order one> is a plausible model; with the sampling interval as the time unit this is an $\operatorname{ARMA}(0,1)$ model.
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