The source producing sound and an observer both are moving along the direction of propagation of sound waves. If the respective velocities of sound, source and an observer are v, v_s and v₀, then the apparent  frequency heard by the observer will be (n = frequency of sound)

• A

  $\frac{\mathrm{n}(\mathrm{v}+{\mathrm{v}}_{\mathrm{o}})}{\mathrm{v}-{\mathrm{v}}_{\mathrm{o}}}$

• B

  $\frac{\mathrm{n}(\mathrm{v}-{\mathrm{v}}_{\mathrm{o}})}{\mathrm{v}-{\mathrm{v}}_{\mathrm{s}}}$

• C

  $\frac{\mathrm{n}(\mathrm{v}-{\mathrm{v}}_{\mathrm{o}})}{\mathrm{v}+{\mathrm{v}}_{\mathrm{s}}}$

• D

  $\frac{\mathrm{n}(\mathrm{v}+{\mathrm{v}}_{\mathrm{o}})}{\mathrm{v}+{\mathrm{v}}_{\mathrm{s}}}$