He had kept up his training regimen for a month. The given sentence is
Right Answer is: D
The given sentence is of past perfect tense and it is in active form. The structures for active/passive voices are:
Active: Subject + had + verb (IIIrd form) + object…
Passive: Object + had + been + verb (IIIrd form) + by + subject…
So, the passive voice of the given sentence would be:
His training regimen had been kept up by him for a month.