Knowing when to use "less" and "fewer" is often something people find difficult. which of the following sentences is wrong?

a. i get fewer hours of sleep these days.
b. i ate fewer apples today than yesterday.
c. there's less apples in the garden today.
d. i cycled less than three miles today.

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.