The following equation represent progressive transverse waves
$$
\begin{aligned}
&z_{1}=A \cos (\omega t-k x) \\
&Z_{2}=A \cos (\omega t+k x)
\end{aligned}
$$
$z_{3}=A \cos (\omega t+k y)$
$z_{4}=A \cos (2 \omega t-2 k y)$
A stationary wave will be formed by superposing
(a) $z_{1}$ and $z_{2}$
(b) $z_{1}$ and $z_{4}$
(c) $z_{2}$ and $z_{3}$
(d) $z_{3}$ and $z_{4}$