The invariant interval is defined by
$\begin{align*}(\Delta s)^2 \equiv (\Delta x)^2 + (\Delta y)^2 + (\Delta z)^2 - c^2(\Delta t)^2\end{align*}$
Plug in $\Delta x = 2$ , $\Delta y = 0$ , $\Delta z = 2$ , $\Delta t = 2$ , and $c = 1$ to find that
$\begin{align*}(\Delta s)^2 = 4 + 4 + 0 - 4 = 4 \Rightarrow \Delta s = 2\end{align*}$
Hence answer (C) is correct.

Solution to 1986 Problem 21