Under which pardoning power of the President, duration of sentence is reduced without changing its character?

Answer: Option B

Under Article-72 of the constitution, The President of India is authorised to grant pardon, reprieve, respite or remission of punishment.
Remission is reduction in prisoner's sentence without changing the character or nature of punishment.
Commutation - Changing the character of punishment.
Reprieve - Its a temporary relief or postponment of punishment, specially in death sentence.
Respite - Awarding lesser sentence than prescribed because of special circumstances eg. Pregnancy of women offender.