Pyrolusite is an ore of:

manganese

The correct answer is option 2. manganese.

Pyrolusite is a mineral and the most important ore of manganese. It is a manganese dioxide mineral \((MnO_2)\) and is commonly found in deposits alongside other manganese minerals. Manganese is used in the production of steel, batteries, ceramics, and other industrial applications. Therefore, pyrolusite is primarily mined for its manganese content.