N.
What is longitudinal wave?
0.
Define transverse wave.

Longitudinal waves are those waves which move in the same direction as of the particle of the medium. That means, The Particles of the medium move forward and backward just like the Wave move forward so they propagate in the same direction.

The Sound wave is the example of the longitudinal wave as it propagates in  the same direction as the particle do.

Transverse wave are those waves which propagate in the perpendicular direction as of the particle of the medium. That means, The particle moves up and down but the wave propagate in forward direction. So they dont move in similar direction but perpendicular.

Water wave is the perfect example of the transverse wave as you can see that water particles form crest(Up the midline) and trough(down the midline) i.e. upward and downward movement but the water actually moves forward in direction.

d) the number of neutrons plus the number of protons

the answer is to check all of them! they all apply well at least i think so!