Which of the following options represents an invalid well$-$formed formula in predicate logic?

Group of answer choices

$$x $($P$($x$)->($Q$($x$)$y $($R$($x$,$ y$))))$

$$x $($y $($P$($x$)$ Q$($y$))->$z $($R$($x$,$ z$)$ S$($y$,$ z$)))$

$$x $($P$($x$)$x $($Q$($x$)))$

$$x $($P$($x$)->($y $($Q$($y$)$ R$($x$,$ y$))))$