The mother-in-law buys food for her daughter-in-law. How should I reply to her?

You should thank your mother-in-law. Don't think your mother-in-law deserves it. You should say, oh, mom, thank you for buying me so many delicious foods. I'll do it in the kitchen, mom. You eat first. You can also exercise when you go shopping. When you are old and can't walk, I will also buy food and cook for you and honor you like my parents.