Data of electronegativity are based on the following webpage :
https://en.wikipedia.org/wiki/Electronegativities_of_the_elements_(data_page)
Electronegativities :
C = 2.55, Si = 1.90, O = 3.44, Cl = 3.16, I = 2.66, Br = 2.96, As = 2.18
Difference in electronegativity between C and Si = 2.55 - 1.90 = 0.65
Difference in electronegativity between O and Cl = 3.44 - 3.16 = 0.28
Difference in electronegativity between I and Br = 2.96 - 2.66 = 0.30
Difference in electronegativity between As and Cl = 3.16 - 2.18 = 0.98 (the greatest)
The greater the difference in electronegativity between two elements, the more polar the bond that formed by the atoms of these two elements is.
Hence, As-Cl bond is the most polar.