Slowing rotation reduces EMF frequency Steps:

• Induced EMF in AC generator follows Faraday's law, producing sinusoidal waveform with frequency proportional to magnet's rotation speed.
• Normal rotation yields standard frequency trace on oscilloscope.
• Slowing magnet decreases rotation speed, lowering frequency (longer period between peaks).
• Resulting trace shows stretched oscillations, matching lower frequency option.

Why D is correct:

• D displays reduced frequency waveform, as EMF frequency $f = \frac{n p}{60}$ (where $n$ is speed in rpm, $p$ is pole pairs) decreases with slowing speed.

Why the others are wrong:

• A: Higher frequency indicates faster rotation, opposite effect.
• B: Standard frequency matches normal speed, not slowed.
• C: Distorted shape suggests irregular motion, not uniform slowing.

Final answer: D