Which part is the tail of the letter "r"?

Solution 1:

In typography, the tail is the descending, often decorative stroke on the letter Q or the descending, often curved diagonal stroke on K or R. The descender on g, j, p, q, and y are also called tails.

Here's a quote from Eric Gill (https://en.wikipedia.org/wiki/Eric_Gill) in his Essay on Typography:

• A typically moral and conscientious Englishman finds it exceedingly difficult to keep morals out of art talk; he finds himself inclined to think e.g. that R ought to have a bow more or less semi-circular and of a diameter about half the height of the stem, & a strongly outstanding tail; that an R with a very large bow and hardly any tail at all is wrong.

Pages 50 and 51 of his Essay illustrate his point with all kinds of faulty tails on Rs: https://monoskop.org/images/8/8d/Gill_Eric_An_Essay_on_Typography.pdf