It is generally seen that, when a substance absorbs heat energy, its heating curve shows two plateaus. Which phenomenon is indicated by these plateaus?

The correct answer is;

Change of phase.

Explanation:

When the physical state in which a substance exists in nature changes to another physical state such as a change from solid state to a liquid stat or from a liquid state to a gaseous state, the substance is said to have undergone a phase change, or phase transformation.

The state of a substance will change, depending on the surrounding temperature and the applied pressure and the energy required for the substance to change its state is known as latent heat.

The observed plateaus are periods the latent heat is absorbed to completely change the phase of the substance.