Explanation:

- Batholiths are large masses of igneous rock that form deep underground. They are exposed at the Earth's surface after the overlying rock erodes away.

- Option 1: Groundwater

- Not applicable. Batholiths are not formed by the action of groundwater. Groundwater typically affects sedimentary formations.

- Option 2: Fluvial action

- Not applicable. Fluvial action relates to rivers and streams, which carve landscapes but do not create batholiths.

- Option 3: Glaciers

- Not applicable. Glaciers shape landscapes by erosion and deposition, not by forming batholiths.

- Option 4: Volcanic activities

- Correct. Batholiths form from the cooling of molten magma deep underground, associated with volcanic activity.