In the following question, a sentence has been given in Active/Passive voice. Out of the four alternatives suggested, select the one which best expresses the same sentence in Passive/Active voice. He had kept up his training regimen for a month.

• Keeping up the training regimen had been done by him for a month.
• For a month he has had been keeping his training regimen.
• He is training and this has been kept for a month.
• His training regimen had been kept up by him for a month.