Final answer:
Reserves must be identifiable, technologically accessible, and economically viable to mine. 'All rock types likely to host undiscovered ore' do not fit the definition of reserves, which are known and presently exploitable minerals or ores.
Step-by-step explanation:
When discussing reserves, by definition, they must be identified ore deposits, exploitable with current technology, and economically profitable to mine at present. The option that does not fit into the definition of reserves is d all rock types that are likely to host as yet undiscovered ore. Reserves pertain specifically to known resources that can currently be extracted and are financially viable. Rock types that may host undiscovered ore are considered part of the geological potential or prospect, but they are not classified as reserves until they are proven to be economically and technically viable.
Technological advances can sometimes make previously impractical resources available, therefore adding to the known reserves. Continual exploration also contributes to reserves but with increasingly diminishing returns due to the finite nature of Earth's resources and the easier sources having been exploited first.