What has Jem Finch left behind in the novel? Support the answer with a quote from the text.

This question checks on your understanding of the story. The answer is that Jem leaves his pants behind when he gets them caught on a fence. 

You can find this part of the story in Chapter 6:

"Jem held the bottom wire; Dill and I rolled through and were halfway to the shelter of the schoolyard’s solitary oak when we sensed that Jem was not with us. We ran back and found him struggling in the fence, kicking his pants off to get loose. He ran to the oak tree in his shorts."

In the Warner Books version of this book (the one that has the bird and the tree on the cover,) that quote is on pages 53-54. But you can find the quote in any version of the book by flipping to Chapter 6, then looking at right about the middle of that chapter. 

Of course, once you've determined that Jem's pants got snagged on a fence and that Jem continued escaping with Scout and Dill in just his underwear, which is hilarious, you'll also want to know why they were running away in the first place!

They had been sneaking up on Boo Radley's house, wanting to get a peek at him through the window. But then someone inside the house seems to see the kids and starts approaching them. They freak out and run away all helter-skelter, back through the tight squeeze of the fence, the same way they came in--and that's how Jem lost his pants, and some of his dignity, too, when he had to show up at home without them!