"Are you a boy or a girl?"
Translation:Jsi kluk nebo holka?
Kluk and chlapec are pretty interchangeable, kluk is more colloquial, I'd say. Hoch is quite formal. Mladík means a young man. Holka and dívka are pretty interchangeable, though dívka sounds more formal. Děvče is pretty old-fashioned. (Do note that děvče is a neuter, the same as German 'das Mädchen', the other words are masculine or feminine respectively.
'Děvče' and 'dívka' are totally interchangeable, just keep in mind that 'děvče' is neuter and 'dívka' is feminine. 'Holka' is more colloquial than both of them.
'Chlápek' is NOT a boy but "a guy / some guy".
'Chlapec' is more formal than 'kluk'.
'Mladík' means "young man / youngster".