Doppler Effect in Sound Steps:

• Doppler effect raises observed frequency when sound source moves toward stationary observer.
• Lowers frequency when source moves away.
• Swing's motion creates radial velocity component toward or away from man.
• Highest frequency occurs with maximum approach speed toward man, at position Z.

Why C is correct:

• At Z moving toward man, source velocity directly approaches observer, maximizing frequency shift per Doppler formula $f' = f \frac{v}{v - v_s}$ where is toward observer.

Why the others are wrong:

• A: At X moving away, source recedes, lowering frequency.
• B: At Y moving away, source recedes (likely bottom position), lowering frequency.
• D: Identical to C, but phrasing redundant; C is standard choice.

Final answer: C