Which statement about light waves and sound waves is correct?
A. Both light waves and sound waves show the Doppler effect.
B. light waves can be diffracted but sound waves cannot be diffracted
C. Sound waves are transverse waves and light waves are longitudinal waves
D. sound waves can travel in a vacuum but light waves can travel in a vacuum.

Hint: You could approach each point one by one. You could then recall which of the properties are exhibited by sound waves and which by light waves. Thus you could easily determine the true statements and false statements from the given ones. You could also recall the experiments done in your lower classes to make it easier.

Complete answer:
In the question we are given certain properties of light waves and sound waves and we are asked to find which among the given statements are correct. Let us approach each of the points one by one.
For the case of Doppler effect:
It is one such phenomenon that is exhibited by all types of waves and is notable in sound waves, light waves and also water waves. This effect occurs as the result of relative motion between the source and the observer. It is defined as the apparent downward and upward shift in frequency when the source and observer are receding and approaching respectively. So, option A is found to be true.
For the case of diffraction:
Both light waves and sound waves are known to bend around the corners of any obstacle or through any aperture into the geometrical shadow region of this obstacle or aperture, that is, both show diffraction. Hence, option B is wrong.
Option C is also wrong as we know that light waves are transverse and sound waves are longitudinal.
You could recall the bell-jar experiment from your lower classes to know that sound waves don’t travel in vacuum. But light waves do travel in vacuum. Hence, option D is also false.

So we find that option A is the only true statement here.

Note:
In questions like this where we are to find the true statements from many given statements, we should always consider the possibility of multiple options being true. As sound waves propagate by pressure variation a propagating medium is must for them. Also, sound waves are mechanical waves and light waves are electromagnetic.