Doppler Effect for Approaching Source Steps:

• Apply the Doppler formula for a source moving toward a stationary observer: f' = f × (v / (v - v_s)).
• Insert values: f = 40 Hz, v = 340 m/s, v_s = 25 m/s.
• Find v - v_s = 340 - 25 = 315 m/s.
• Calculate f' = 40 × (340 / 315) ≈ 40 × 1.092 = 43.7 Hz.

Why D is correct:

• The Doppler formula f' = f × (v / (v - v_s)) directly yields 43.7 Hz for an approaching source, increasing the recorded frequency.

Why the others are wrong:

• A: Uses receding source formula f × (v / (v + v_s)) ≈ 37.0 Hz.
• B: Applies incorrect double shift, like for echo from receding reflector.
• C: Relies on low-speed approximation f × (1 + v_s / v) ≈ 43.0 Hz, ignoring higher-order terms.

Final answer: D