The app says <hr />, will work... but I have studied that <hr>... </hr> is correct... Which one is correct?

Answer 1

all of this will work correctly <hr>, <hr/>, <hr/>.
the slash is needed when using xhtml, in html you are not needed to close <hr> element. this is a self closing element.

you can check for yourself this one works '<hr/>' & '<hr>, '<hr />'. but not this one '</hr>'

Answer 2

<hr/> and <hr> will work, but don't try to close it, it's an empty tag...