Best telephoto lens for pentax k?

Here is what the site says about that.

From Pentax K: Pentax K: Pentax's DSLR mount since 1975: decades of lenses still fit the current bodies, which have stabilisation built in. Old manual K lenses fit directly.

From Pentax K-500: Pentax K-500: A DSLR by Pentax with a 16.3-megapixel APS-C sensor, announced in 2013. The K-50 without weather sealing.

Quoted from this site's pages.