Peaks are the highest points of a wave and are called crests, while valleys are the lowest points of a wave and are called troughs.

Compressions are regions where both density and pressure are high, while rarefactions are regions of low pressure where particles are spread apart.

The density and pressure variations in the graphic representation of a sound wave illustrate how these properties change with distance as the wave propagates through the medium, showcasing the compressions and rarefactions within the wave.

In the graphic representation of a sound wave, pressure variations are typically depicted by fluctuations of the curve above and below the average pressure value, similar to density variations.

Graph (c) represents how pressure changes as a sound wave moves through the medium, with fluctuations occurring above and below the average pressure value.