Which sentence shows the correct use of a common homophone?

The art teacher thought you're drawing was exceptionally detailed.

Some of the neighbors just saw you're dog running down the street.

I think you left your biology notebook on the bleachers yesterday.

If you want to finish the assignment, your going to need another pen.

Let's go though all the sentences:

1) The art teacher thought you’re drawing was exceptionally detailed.

here it should be "your", not "you're" - it's possessive

2) Some of the neighbors just saw you’re dog running down the street.

here it's also possessive: your dog, not you're dog

3) I think you left your biology notebook on the bleachers yesterday.

-this is the correct sentence! it's a possessive form and it needs to be a possessive form

4) If you want to finish the assignment, your going to need another pen.

Here the possessive form is wrong! it should be "you're going"