The correct answer is $\boxed{\text{B) 45°}}$.
The resultant of two forces is the single force that would produce the same effect as the two forces acting together. The magnitude of the resultant force is equal to the square root of the sum of the squares of the magnitudes of the two forces. The angle between the two forces is the angle between the directions of the two forces.
If the resultant of two forces has the same magnitude as either of the force, then the two forces must be equal in magnitude and must be acting at right angles to each other. This is because the square of the hypotenuse of a right triangle is equal to the sum of the squares of the other two sides. Therefore, if the two forces are equal in magnitude and are acting at right angles to each other, then the resultant force will also be equal in magnitude.
The other options are incorrect because they do not represent right angles. Option A, 30°, is incorrect because the resultant force would be less than either of the forces. Option C, 60°, is incorrect because the resultant force would be greater than either of the forces. Option D, 120°, is incorrect because the resultant force would be zero.