Answer: C. Q is longitudinal because its particles oscillate parallel to the direction of travel. Since the two waves have the same speed, $v=f\lambda=(2f)\lambda_Q$ so $\lambda_Q=\frac{\lambda}{2}$

A: reverses the wave classification and treats wavelength as directly proportional to frequency.
B: finds the wavelength correctly but labels parallel oscillations as transverse.
D: identifies the wave correctly but uses a direct rather than inverse frequency–wavelength relationship.

Remember: at fixed wave speed, frequency and wavelength are inversely proportional.