What is this end part called? What adapter do I need to attach a tank?
That is called a flare fitting. It mates to a roughly cone shaped fitting (usually on a valve) which makes a metal on metal seal with pressure when the nut is screwed down on threads on the fitting. No tape or pipe dope needed or would do any good. The flared tubing is soft copper.

The size would be the OD of the copper tubing which looks like 1/2 inch or 3/8 inch.