Which extender do I buy for my AC1600 WiFi VDSL/ADSL Router model D6400?

A very loaded statement since there are so many options and you didn't give us much info to go off. 

if you go to the netgear extender page it'll have a list of all the extenders. Theres options on the left that you can click to filter them by area you're wanting covered, ethernet ports desired, wireless specs you want to meet. this should filter it down so you can select from a few devices versus many devices.