Step 1: Use the Pythagoras theorem.
In a right triangle, the square of the hypotenuse is equal to the sum of the squares of the other two sides. Let the perpendicular side be \( x \). Then according to the Pythagoras theorem:
\[
\text{Hypotenuse}^2 = \text{Base}^2 + \text{Perpendicular}^2
\]
\[
7.5^2 = 4.5^2 + x^2
\]
\[
56.25 = 20.25 + x^2
\]
\[
x^2 = 56.25 - 20.25 = 36
\]
\[
x = \sqrt{36} = 6 \, \text{cm}
\]
Thus, the measure of the perpendicular side is 6 cm. The correct answer is (B).