1. Use the basic wave relation

   For any wave, $v = f\lambda$ where:

   • $v$ = wave speed
   • $f$ = frequency
   • $\lambda$ = wavelength

2. What happens on refraction into a denser medium?

   When a wave enters a denser medium, its speed decreases.

3. What happens to frequency?

   The frequency is determined by the source, so during refraction, $f = \text{constant}$

4. Find the effect on wavelength

   Since $v = f\lambda$ and $f$ remains constant, if $v$ decreases then $\lambda$ must also decrease.

   So in the denser medium:

   • speed decreases
   • frequency remains constant
   • wavelength decreases

5. Check the options

   • A: wavelength, speed and frequency decreases. ❌ Frequency does not decrease.
   • B: wavelength increases, speed decreases and frequency remains constant. ❌ Wavelength does not increase.
   • C: wavelength and speed decreases but frequency remains constant. ✅ Correct.
   • D: wavelength, speed and frequency increases. ❌ Incorrect.

Final Answer

The correct option is: $\boxed{\text{C}}$