Aright triangle has 23 mm, 20 mm, and 13 mm but is it a scalene, isosceles, or a

-- It can't be a right triangle, because the squares of the 2 short sides
don't add up to the square of the longest side.

-- It can't be isosceles, because every isosceles triangle has 2 sides
the same length.

-- It can't be equilateral, because every equilateral triangle has all 3 sides
the same length.

-- So, out of the choices you've listed, this is a scalene triangle.
A scalene triangle is simply one whose sides are all different lengths.