Concept:In indirect speech, when the reporting verb is in the past tense, the tense of the reported clause generally shifts one step back. Time and place words like "this" change to "that". The modal "must" can remain unchanged if it expresses a strong obligation.
Rule / Clue:The reporting verb "said" is past tense, so the simple past "did" becomes past perfect "had done". The demonstrative "this" becomes "that". The original passive construction "must be held accountable" should be preserved without changing the meaning.
Explanation:Option A is incorrect because it keeps the simple past "did" and changes "this" to "it" instead of "that".
Option B is incorrect because it replaces "whoever" with "the one who" and uses "must be accounted", which changes the original meaning.
Option D is incorrect because it retains "this" and uses "must have held accountable", which distorts the passive voice and grammar.
Option C correctly applies all rules: "did" becomes "had done", "this" becomes "that", and "must be held accountable" remains unchanged, accurately reflecting the original statement.
Answer:He said that whoever had done that must be held accountable. (Option C)