1. During a conversation with Meg (while building a kite for the twins) during which Jo indicates regret at not having accepted Laurie’s marriage proposal, Jo and Meg both compare Jo to what?

Both liken Jo to a "chestnut burr".

"It's just what you need to bring out the tender womanly half of your nature, Jo. You are like a chestnut burr, prickly outside, but silky-soft within, and a sweet kernal, if one can only get at it. Love will make you show your heart one day, and then the rough burr will fall off."
"Frost opens chestnut burrs, ma'am, and it takes a good shake to bring them down. Boys go nutting, and I don't care to be bagged by them," returned Jo, pasting away at the kite which no wind that blows would ever carry up, for Daisy had tied herself on as a bob.