Answer:
D.The appearance of flame and fire on the hound.
Explanation:
From The Hound of the Baskervilles by Sir Conan Doyle, Sherlock Holmes and Dr Watson go to solve a mystery of the death of Sir Baskerville.
Sherlock and his team have an encounter with the large hound and manage to shoot it before it could injure or kill anyone else.
On close inspection, they discover that someone covered the animal in a clever preparation of phosphorus and Watson observes that "Fire burst from its open mouth, its eyes glowed with a smouldering glare, its muzzle and hackles and dewlap were outlined in flickering flame".
So, the appearance of flames on the hound was due to the phosphorous.
