If a wave gets refracted into a denser medium, which of the following is true?

frequency remains the same but both wavelength and speed decrease

The correct answer is Option (4) → frequency remains the same but both wavelength and speed decrease

$\text{When a wave enters a denser medium:}$

$\text{Frequency remains constant: } f = \text{constant}$

$\text{Speed decreases: } v_\text{new} < v_\text{old}$

$\text{Wavelength decreases: } \lambda_\text{new} = \frac{v_\text{new}}{f} < \lambda_\text{old}$

$\text{Answer: } \text{frequency remains the same but both wavelength and speed decrease}$