Can someone help? Explain and give work in how you got the answer.

[tex]▪▪▪▪▪▪▪▪▪▪▪▪▪ {\huge\mathfrak{Answer}}▪▪▪▪▪▪▪▪▪▪▪▪▪▪[/tex]
We know the side relations of a right angled Triangle (that is Pythagoras therom)
now, let the missing side be x
that is ~
The correct choice is ~ B