Which of these is the right <br>, <br/> or <br /> ?

Answer 1

<br> is enough for HTML 5, but you can also use <br/> and <br /> for compatibility with XHTML.
You should prefer <br /> in XHTML.
https://www.w3.org/TR/xhtml1/#C_2

Answer 2

React.js uses <br />

Answer 3

All options are correct for HTML5 and can be used in code.

Answer 4

<br /> will work all old version. but you can use any of them for modern browser