Final answer:
The most ionic bonds are Na-O, K-Cl, and Fr-F because they have the greatest differences in electronegativity between their respective atoms, which is a key determinant of ionic character in a bond.
Step-by-step explanation:
To determine which bond from each set is most ionic, we look at the difference in electronegativity between the atoms in each bond. The greater the difference, the more ionic the bond is. For the set Al-O and Na-O, Na has a lower electronegativity than Al, and thus Na-O is more ionic. For the set K-Cl and Zn-Cl, although Zn and K have similar electronegativities, K is an alkali metal which typically forms more ionic bonds than transition metals like Zn, making K-Cl more ionic. For the set Fr-F and B-F, Fr is an alkali metal and has a very low electronegativity compared to B, which is a metalloid. Therefore, Fr-F is the most ionic due to the larger electronegativity difference between Fr and F.