A source of sound moves toward a stationary observer at a speed of 34 m/s. If the speed of sound in air is 340 m/s and the frequency of the source is 500 Hz, what frequency does the observer hear?

Using the Doppler formula for a source moving toward a stationary observer: $f' = f \times \frac{v}{v - v_s} = 500 \times \frac{340}{340 - 34} = 500 \times \frac{340}{306} \approx 500 \times 1.111 \approx 555.6 \approx 556$ Hz. Option C (550 Hz) is a common error from using $v + v_s$ in the denominator incorrectly, while A results from applying the formula for a receding source.