The active voice sentence is "He had kept up his training regimen for a month." Choose the option that best expresses this sentence in the passive voice.

Introduction / Context:
This question checks the learner's ability to convert a past perfect tense sentence in active voice into an appropriate past perfect passive voice form. The original sentence emphasises the subject "he" who consistently maintained his training regimen for a specified period. In the passive, focus moves to "his training regimen," while the time reference and duration are preserved.

Given Data / Assumptions:

• Active sentence: He had kept up his training regimen for a month.
• Subject: he.
• Object: his training regimen.
• Verb phrase: had kept up (past perfect of "keep up").
• Time phrase: for a month, indicating duration of the action.

Concept / Approach:
The passive form of the past perfect tense uses the structure: subject (former object) + had been + past participle. Here, "his training regimen" becomes the subject of the passive sentence. The phrasal verb "had kept up" changes to "had been kept up." We then add "by him" as the agent phrase to indicate who maintained the regimen. The time phrase "for a month" remains unchanged to show duration.

Step-by-Step Solution:

Step 1: Identify the object "his training regimen" that will become the subject in the passive sentence. Step 2: Note the tense "had kept up," which is past perfect. In passive form, this becomes "had been kept up." Step 3: Insert the agent phrase "by him" after the verb phrase to indicate who performed the action. Step 4: Retain the time phrase "for a month" at the end to show how long the regimen was maintained. The final passive sentence is: "His training regimen had been kept up by him for a month."

Verification / Alternative check:
The sentence "His training regimen had been kept up by him for a month" keeps the original meaning that the regimen was maintained during a past period before another point in the past. It correctly uses past perfect passive: "had been kept up." The focus has shifted from "he" to "his training regimen," which fulfils the requirement of changing from active to passive voice.

Why Other Options Are Wrong:

Option B: "Keeping up the training regimen had been done by him for a month." is grammatically heavy and unnatural, using an awkward nominalisation and the phrase "had been done" instead of simply "had been kept up." Option C: "For a month he has been keeping up his training regimen." uses present perfect continuous, which changes the time reference from a completed past period to an ongoing or recent action. Option D: "He is training and this has been kept for a month." does not reflect past perfect and introduces a new clause, thereby changing the meaning. Option E: "His training regimen was kept up by him for one month now." mixes simple past with "now," which is inconsistent and does not match the past perfect nature of the original sentence.

Common Pitfalls:
Learners may forget to use "been" in perfect passive forms or may change the tense to simple past or present perfect instead of staying with past perfect. Another common mistake is rewriting the sentence in a very clumsy way with "had been done," which is rarely necessary when a direct passive form of the original verb exists. Always keep the original tense and aspect, then apply the correct passive structure "had been + past participle."

Final Answer:
The correct passive voice sentence is His training regimen had been kept up by him for a month.