Doppler-like effect for moving wave source

Steps:

• Stationary insect produces symmetric circular waves with constant frequency and speed at X.
• Insect moving right compresses waves ahead (at X, assuming forward position), increasing crest frequency.
• Motion adds source velocity to wave propagation, raising effective wave speed at X.
• Result: higher frequency and faster speed match observed bunching and advance.

Why D is correct:

• Moving source toward observation point increases frequency (f' > f) and adds velocity component, boosting wave speed per wave propagation principles.

Why the others are wrong:

• A: Frequency increases, not decreases, due to wave compression.
• B: Speed stays constant in medium, but motion increases effective speed; frequency rises.
• C: Speed unchanged, but insect motion increases effective speed at X.

Final answer: D