Which covalent bond is the most polar and why?
h-ci h-br h-s h-c f-f

H-Cl is more polar than other because of greater electronegativity difference between chlorine and hydrogen.

Explanation:

Covalent bond:

It is formed by the sharing of electron pair between bonded atoms.  

The atom with larger electronegativity attract the electron pair more towards it self and becomes partial negative while the other atom becomes partial positive.

For example:

In water the electronegativity of oxygen is 3.44 and hydrogen is 2.2. That's why electron pair attracted more towards oxygen, thus oxygen becomes partial negative and hydrogen becomes partial negative and polar covalent bond is formed.

when the electronegativity difference is above the 0.4 polar covalent bond is formed. While if it is 0.4 or less than 0.4 non polar covalent bond is formed.

H-Cl

3.16 - 2.2 =0.96

H- Br

2.96 - 2.2 = 0.76

H-S

2.58 - 2.2 = 0.38

H-C

2.55 - 2.2 = 0.35

F-F

3.98-3.98 = 0

H-Cl is more polar than other because of greater electronegativity difference between chlorine and hydrogen.

i would say b

explanation:

but i think i would need more information than that

a electronegativity decreases beacuse the size of nucleus is bigger.