Ionic bond is formed due to the transfer of electrons from one atom to the other. In this bond, one atoms (the donor) loses (donates) one or more electron(s) to the other atom (the acceptor) in order for both to become stable.
Examining the choices we have, we will find that K needs to lose one electron while Br needs to gain one electron in order for both to be stable. Therefore, K loses one electron that is gained by Br.
Based on this, the bond in KBr is considered ionic.
The right choice is:
K and Br
