While Tom was watching TV, he smelled something ____.
(A) burning (B) to burn (C) burned (D) was burning
The answer is option A. Is option C acceptable?
B is wrong.
C would be right only if there were some context to make 'burned' work as a past participle rather than a simple past: 'He smelled something burned to the bottom of the pan'. [This example is a bit dubious, even to me - I think in this case the speaker would probably emphasize the adhesion more, by saying something like 'He smelled something burning, stuck to the bottom of the pan' or 'He smelled something burned on to the bottom of the pan'.]
D is a common colloquial usage, with the elision of 'that': "He smelled [that] something was burning.
So, in an exam, use A, but don't be surprised if in real life you hear D too.