The number of lines of symmetry in a 45° - 45° - 90° set-square is

As we know that, the 45° - 45° - 90° set-square is of the shape of an isosceles triangle, and we know that the isosceles triangle has only one line of symmetry, so we can say that 45° - 45° - 90° set-square has 1 line symmetry.

∴ the correct option is B.