For specular reflection of a light ray, the angle of incidence a) must be equal to the angle of reflection. b) is always less than the angle of reflection. c) is always greater than the angle of reflection. d) is equal to \(90^{\circ}\) - the angle of reflection. e) may be greater than, less than, or equal to the angle of reflection.

Specular reflection occurs when a parallel beam of light rays hits a smooth surface, and it reflects these rays such that they remain parallel after reflection. In this type of reflection, the angle between the incident ray and the reflected ray is important.

The law of reflection states that for a smooth surface, the angle of incidence (the angle between the incident light ray and the surface normal) is equal to the angle of reflection (the angle between the reflected light ray and the surface normal). Mathematically, this can be written as: \(angle~of~incidence = angle~of~reflection\)

Now, we need to observe the given statements and compare them with the law of reflection: a) must be equal to the angle of reflection. — This statement is true according to the law of reflection. b) is always less than the angle of reflection. — This statement is false, as it contradicts the law of reflection. c) is always greater than the angle of reflection. — This statement is false, as it contradicts the law of reflection. d) is equal to \(90^{\circ}\) - the angle of reflection. — This statement is false because according to the law of reflection, the angle of incidence is equal to the angle of reflection, not their difference. e) may be greater than, less than, or equal to the angle of reflection. — This statement is incorrect since, according to the law of reflection, the angle of incidence must always be equal to the angle of reflection.

Based on our analysis, we can conclude that the correct answer is (a) the angle of incidence must be equal to the angle of reflection for a specular reflection of a light ray.