The heating of ore in absence of air below its melting point is called

Calcination is the process of converting a concentrated ore into its oxide by heating it below its melting point without the presence of air.

On the other hand, Roasting is the process of converting a concentrated ore into its oxide by heating it below its melting point in the presence of air. 

Leaching is done to remove impurities like silicon dioxide $\left({\mathrm{SiO}}_2\right)$, iron oxides, and titanium dioxide $\left({\mathrm{TiO}}_2\right)$ from bauxite. Bauxite is the ore of aluminum. In this process, powdered ore is treated with a concentrated solution of NaOH at 473-523 K and 35-36 bar pressure. NaOH reacts with aluminum oxide $\left({\mathrm{Al}}_2{\mathrm{O}}_3\right)$ and forms sodium aluminate. This sodium aluminate is removed leaving behind the impurities.

The process of obtaining a metal either as an element or as a simple compound from its ore by heating above its melting point in the presence of oxidizing agents like air and coke is known as smelting.

Hence, the correct option is (D) Calcination.