Final answer:
Constructive interference for two waves occurs when one wave is shifted by a full integer multiple of wavelengths, referred to as 'm'. This causes the waves to be in phase and their amplitudes to combine constructively.
Step-by-step explanation:
Constructive interference occurs when two waves combine to create a wave with a larger amplitude. This happens when the waves are in phase, meaning that the crests and troughs of the waves align. For two identical waves, pure constructive interference is obtained when the waves are shifted by a distance that corresponds to an integer multiple of wavelengths, represented by the variable 'm'. This aligns the crests of one wave with the crests of the other wave, resulting in an amplitude increase. The correct answer to the student's question is therefore d) A full integer multiple of wavelengths: m.