If two forces of 3 kg and 4 kg act at right angles to each other, their resultant force will be equal to A. 7 kg B. 1 kg C. 5 kg D. None of these

The correct answer is $\boxed{\text{C}}$.

The resultant force of two forces acting at right angles to each other is equal to the square root of the sum of the squares of the two forces. In this case, the two forces are 3 kg and 4 kg, so the resultant force is equal to $\sqrt{3^2 + 4^2} = \sqrt{25} = 5$ kg.

Option A is incorrect because 7 kg is not the square root of 25.

Option B is incorrect because 1 kg is not the square root of 25.

Option D is incorrect because 5 kg is the square root of 25.