The least polar bond is found in a molecule of
1. hi
2. hf
3. hci
4. hbr

1. HI

Explanation:

Electronegativity difference gives the magnitude of the bond polarity

Element   Electronegativity

H                2.1

I                  2.5

F                 4

Cl                3

Br                2.8

Compound     Polar bond

HI                        2.5 - 2.1 = 0.4

HF                       4 - 2.1 = 1.9

HCI                     3 - 2.1 = 0.9

HBr                    2.8 - 2.1 = 0.7  

i think it's letter a.