Question

All rectangular pentagons are similar to each other true or false

Answer 1

true, all rectangular pentagons are similar to each other

 

Answer 2

Rectangular pentagons don't exist, a shape can be either a rectangle or a pentagon but not both, so for this answer, I'm going to assume you meant "regular" =P

If you did indeed mean regular, then true, all regular pentagons are similar to each other. Regular means that the shape has all angles of equal measure and all sides of the same length. Similar shapes have the same angle measures as each other, but the side length may be variable. So basically, for similar shapes are either shrunken or magnified versions of each other.
Hope that helped =)