Based on the given information, we can use the Pythagorean theorem to find the length of BC. Since AC and DE are perpendicular to each other, we can consider them as the legs of a right triangle. Using the theorem, we have AC^2 + DE^2 = BC^2. Substituting the given values, we get 6^2 + 3^2 = BC^2. Simplifying, we have 36 + 9 = BC^2. Therefore, BC^2 = 45. Taking the square root of both sides, we get BC = √45 = √(9 * 5) = 3√5. Since the answer options are given in whole numbers, we can approximate √5 to be around 2.24. Therefore, BC is approximately equal to 3 * 2.24 = 6.72, which is closest to 7 cm. Hence, the correct answer is 7 cm.