Select the correct option that fills the blank(s) to make the sentence meaningfully complete.

The correct preposition is 'to' because the phrase 'reduced him to a skeleton' is an idiomatic expression meaning to make someone extremely thin or emaciated due to illness. The preposition 'to' indicates the end state or condition that results from the illness. The other options do not form grammatically or idiomatically correct phrases in this context.

Step-by-step Derivation:
Analyze the sentence structure: 'The protracted illness has reduced him [preposition] skeleton.' The verb 'reduce' commonly takes 'to' when indicating transformation into a state or condition (e.g., 'reduced to poverty,' 'reduced to tears,' 'reduced to a skeleton'). Testing alternatives: 'Till' (incorrect—not standard usage), 'Round' (incorrect—does not fit), 'Through' (incorrect—does not form a valid phrase). Only 'to' creates the correct idiomatic expression.