1. It sits

Because apostrophes are used for possession as well as contraction, many students get confused about the distinction between it's and its. The solution here is to remember that it's is a contraction, not a possessive: It's = it is. It, like other pronouns （he, his; she, her） doesn't take an apostrophe to show possession:

WRONG RIGHT
The essay's comic tone clashes with it's dark subject.
The essay's comic tone clashes with its dark subject.

　　Who's and whose have the same distinction:

WRONG RIGHT
Sicinius, who's modern equivalent would be a congressman, accuses the old warrior of being a traitor.
Sicinius, whose modern equivalent would be a congressman, accuses the old warrior of being a traitor.