The correct answer is (d) He declared his innocence.
Sentence Transformation — Complex to Simple:
A Complex sentence contains a main clause and one or more subordinate clauses. A Simple sentence has only one clause with a single finite verb.
Analysis:
— Complex: "He said that he was innocent" — has a reported speech clause (that he was innocent).
— To simplify: Replace the reported clause with a noun phrase.
— "he was innocent" → "his innocence"
— "He said" → "He declared"
→ Simple: "He declared his innocence." ✓
✗ Option (a): still compound/complex.
✗ Option (b): changes meaning.
✗ Option (c): passive and awkward, not truly simple.
Source: Wren & Martin; A Practical English Grammar.