The transformation on a line such that a line is obtained which is pependicular to previous line is possible only when it is reflected through y = x axis. This can be understood as,
When line is reflected about x-axis, the x coordinate remain same where as y-coordinate changes in opposite sign with same magnitude. So reflection of line x = 3 about x axis remain same.
Simillarly reflection of line x = 3 about x = 1 remain same as x = 3.
The reflection of line x = 3 about y-axis results n a parallel line passing through x = -3.
So answer is reflection across y = x.