Which of the following sentences demonstrates correct grammar and usage?

The correct answer is A: 'If fewer people watch that television series, the effect will be its cancellation.' In this sentence, 'fewer' is correctly used to refer to countable items, and 'people' is a countable noun. 'Effect' is the correct term to describe the result of fewer people watching the television series. The usage of 'affect' in choices B and C is incorrect; 'affect' is a verb meaning to influence, while 'effect' is a noun meaning result or consequence. Therefore, choice A is the only option that is both grammatically correct and uses the appropriate terms in the context provided.