Its speed increases to 4/3 c, and its frequency decreases.
Its speed decreases to 3/4 c, and its wavelength decreases by a factor of 3/4.
Its speed decreases to 3/4 c, and its wavelength increases by a factor of 4/3.
Its speed and frequency remain the same.
Its speed decreases to 3/4 c, and its frequency increases.