I would say the correct answer is C. There's less apples in the garden today. The difference between "less" and "fewer" is very simple: less is used for uncountable nouns (the ones that can't be counted, e.g. you can't count love, hate, salt, etc.) and fewer for countable (such as apples). So, the correct version of this sentence would be: There are fewer apples in the garden today.