Concept:
In reported speech, a statement in the Present Perfect tense ("have/has + past participle") must be converted into the Past Perfect tense ("had + past participle") when the reporting verb is in the past tense.
Step 1: Identify the tense shift.
The direct speech uses "I have completed," which is the Present Perfect tense. Because the reporting verb is "said," we backshift this to the Past Perfect: "he had completed".
Step 2: Evaluate the options for grammatical errors.
• Option A: Fails to shift the tense (remains "has completed").
• Option B: Uses "had been completed," which is passive voice/past perfect continuous structure and changes the meaning of the original active sentence.
• Option D: Changes the reporting verb from "said" to "says," which is incorrect.
Step 3: Final construction.
The correct form is: He said (reporting verb) + that (conjunction) + he (subject) + had completed (past perfect) + the work. This matches Option (C) perfectly.
Final Answer: Option C