1. C. Wavelength is the length of one complete wave cycle.

In a wave, wavelength is the length of one complete cycle of the wave: it means it is the distance between two points with same shape along the wave. For instance, the wavelength can be measured as the distance between two adjacent crests of the wave, or the distance between two adjacent through of the wave.

2. B. The waves are closer together because the object is moving toward you.

When an object is approaching you, the adjacent wavefronts of the wave becomes closer together, because the source of the wave is moving in the same direction the wave propagates while the wavefronts are emitted. For this reason, the wavelength of the wave appears shorter, and so the frequency increases (frequency is inversely proportional to wavelength), and so the pitch increases (because the pitch is proportional to the frequency).

3. A. Pitch is how frequency is perceived.

In fact, pitch tells how frequency is perceived. For this reason, the pitch of a sound is proportional to its frequency: the higher the frequency of the sound, the higher the pitch, and the lower the frequency, the lower the pitch.