Answer : Compression
Explanation :
An area where the particles in a medium are spaced close together is called a compression. In this region the particles are very close to each other.
In the attached figure, the rarefaction and compression is shown very clearly. In rarefaction, the particles of medium are very far apart form each other.
The distance between two consecutive compression and rarefaction is called the wavelength of wave.
So, the compression is the area where the particles are close to each other.