What extender lens is best for my nikon d40?

What extender lens is best for my nikon d40?

What do you mean by extender lens? Are you after a 1.5x converter to give you a longer focal length or an extension tube for macro photography or do you mean that you want a new telephoto zoom lens to allow you to zoom in closer to the subject?

Whatever you are looking for, whether it be a tele-converter or macro extension tubes, none of these solutions are as good as a lens for that purpose.

While macro extension tubes actually work, you will at some point want a true macro lens.

Extension tubes? Nikon mount preferably the one that preserves auto functions.

Extender lens? Bellows?

Lens adapter? Like tele-adapter or semi-fisheye adapter? The one with the same filter size as your exisitng lens.