The correct answer for the question "The Vindhyas belong to which class of mountains?" is option 2 - Block mountains.

Option 1 - Fold mountains: Fold mountains are formed by the folding of rock layers due to compression forces. Examples include the Himalayas and the Alps. The Vindhyas, however, are not formed by folding of rock layers.

Option 2 - Block mountains: Block mountains are formed by the upliftment of large blocks of the Earth`s crust. These mountains have steep sides and are often bounded by faults or fractures. The Vindhyas are considered to be block mountains as they were formed by tectonic movements and are characterized by their long, east-west extent.

Option 3 - Volcanic mountains: Volcanic mountains are formed by the accumulation of volcanic materials such as lava, ash, and pyroclastic flows. Examples include Mount Fuji in Japan and Mount St. Helens in the USA. The Vindhyas are not formed through volcanic activity.

Option 4 - Residual mountains: Residual mountains are formed by the erosion and weathering of existing landforms. Examples include the Black Forest in Germany and the Appalachian Mountains in North America. The Vindhyas are not considered to be residual