Constructive interference occurs when peaks align with peaks and troughs align with troughs. This results in:

• A. Peaks align with peaks and troughs align with troughs.
• B. Peaks align with troughs.
• C. Peak alignment is unrelated to interference.
• D. Frequencies must be different.

Answer: (A)

Constructive interference happens when waves are in phase, so their displacements add together. When peaks align with peaks and troughs align with troughs, the amplitudes reinforce each other, producing a larger resultant wave (greater amplitude or intensity). That is exactly why this description is correct: in-phase alignment leads to the strongest, additive outcome. If peaks lined up with troughs, they'd partially cancel (destructive interference). The idea that peak alignment is unrelated is incorrect because the interference pattern depends on phase; and requiring different frequencies would prevent a stable, steady constructive peak, since the phase relationship would change over time.