Forthcoming generation will be less adaptive than the present generation due to :

The correct option is C Mutation
According to De Vries theory of evolution:

• Mutation or discontinuous variation is the raw material of evolution.
• The mutation appears suddenly and produced their effect immediately.
• Mutants are different from the parents and there is no intermediate stage between the two.
• The same type of mutation can appear in several individuals of a species.
• Mutation can appear in all direction and all mutations are the inheritance
• Useful mutations are selected by nature an lethal mutations are eliminated.

So, the correct option is 'Mutation'.