The correct answer to the question is option 2. The statement that an unknown amount of coal lies buried under the Ganga alluvium and the Deccan Trap is not correct.

Let`s analyze the other options to have a better understanding:

Option 1 states that 98 percent of the reserves of coal in India belong to the Gondwana period. Gondwana coal refers to the coal deposits that were formed during the Permian and Triassic periods when India was part of the supercontinent called Gondwana. This statement is correct.

Option 3 mentions that bituminous coal is found in both lower Gondwana rocks and in Tertiary rocks. Bituminous coal is a type of coal with a relatively high carbon content and is widely used for energy production. It is indeed found in both lower Gondwana rocks, which are the oldest coal-bearing formations in India, and in Tertiary rocks, which were formed more recently. This statement is correct.

Option 4 states that Gondwana coal is mainly found in the river valleys of Damodar, Mahanadi, and Godavari. Gondwana coal is indeed mainly found in these river valleys in eastern and central India. This statement is correct