Explanation:

The original sentence is erroneous.

Reason: The word ‘obligation’ must be followed the preposition ‘to’ instead of ‘towards’ in this context. The word ‘obligation’ means ‘duty or commitment’.

Hence, the preposition ‘to’ should be used in place of ‘towards’ in order to make the sentence grammatically and contextually correct.

Among the given choices, only option D replaces the given bold part most appropriately.

The sentence after replacement becomes:

Most of us recognise our moral obligation to leave children a decent future, which obviously includes a decent planet.

Option D is hence the correct answer.