Since others seem to be having this issue, I found a tip on a separate comment thread that "hezký" is used in the same way that the English "nice" is used only in certain phrases such as "Hezký vikend." In most other cases it will be used in the way of "pretty" or "cute".
That's why many of the translations get marked incorrect when using "nice" when it's expecting "pretty."
I have tried by clicking my answer should be accepted to suggest that "lad" is a perfectly acceptable translation of "kluk" especially when combined with handsome...to me as a native speaker of British English it sounds much more natural to say "he's a handsome lad" than "he's a handsome boy". Handsome works better with man than boy to my ears...
The "my answer should be accepted" didn't work though.
By the way "pretty boy" is what one traditionally says to a budgerigar... :-)
These colloquial and only regionally used words, like lad, lass... are not accepted in this course. Neither we accept regional words from some parts of the Czech republic like šohaj, galán, děvčica,...
"Handsome works better with man than boy to my ears..." Notice that the main translation uses "pretty boy", not without a reason.