Question

Which of the following sentences contains correct usage of its/it's?
a. let's go out on friday; it's my birthday!
b. the dog chased it's tail until it got dizzy.
c. its only a game.
d. my grandmother always said, "its not if you win or lose, but how you play the game."

Answer 1

The correct answer is A.

It's is short for it is. 

Use its to show possession. For example in sentence B, it's should be its because the dog is the 'owner' of the tail.