Natural backing (woven backing...nothing attached). You want to look for anything that says "Safe for Use with Hardwoods". This is where you START. Do NOT use "rubber" or "natural latex". If you wish to use a rug pad, again you must work with something that says "Safe for Use With Hardwoods". I like https://www.rugpadusa.com/

If you're looking at carpet for area rugs, stay away from jute backed products. (most carpets come with the jute backing) This backing will scuff and scratch your wood floors. Shaw carpets has a 'soft back' and is suitable to lay the carpet directly atop of your wood and will not scratch or scuff it. Dreamweaver carpets is also coming out with soft back for their carpets. Soft back carpets do not need a pad to preserve your wood floors.
If you do find a carpet you love that has a jute back, you will need to purchase a wood friendly pad to go under the carpet.