To determine which of the given options cannot be the sides of a right triangle, we need to apply the Pythagorean theorem. The Pythagorean theorem states that in a right triangle, the square of the length of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the lengths of the other two sides.

Let's analyze each option:

a) 400mm, 300mm, 500mm:
According to the Pythagorean theorem, the square of the longest side (hypotenuse) should be equal to the sum of the squares of the other two sides.
- 400^2 + 300^2 = 160,000 + 90,000 = 250,000
- 500^2 = 250,000

The lengths of the sides in option a) satisfy the Pythagorean theorem, so it can be the sides of a right triangle.

b) 9cm, 15cm, 12cm:
- 9^2 + 15^2 = 81 + 225 = 306
- 12^2 = 144

The lengths of the sides in option b) do not satisfy the Pythagorean theorem since the sum of the squares of the smaller sides is not equal to the square of the longest side. Therefore, option b) cannot be the sides of a right triangle.

c) 2cm, 1cm, 5cm:
- 2^2 + 1^2 = 4 + 1 = 5
- 5^2 = 25

The lengths of the sides in option c) do not satisfy the Pythagorean theorem since the sum of the squares of the smaller sides is not equal to the square of the longest side. Therefore, option c) cannot be the sides of a right triangle.

d) 9cm, 5cm, 7cm:
- 9^2 + 5^2 = 81 + 25 = 106
- 7^2 = 49

The lengths of the sides in option d) do not satisfy the Pythagorean theorem since the sum of the squares of the smaller sides is not equal to the square of the longest side. Therefore, option d) cannot be the sides of a right triangle.

Hence, the correct answer is option 'D' (9 cm, 5 cm, 7 cm).