Pick out the molecule which has zero dipole moment.

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: $NH_3$ and $NF_3$ molecules have a pyramidal shape with a lone pair of electrons on the nitrogen atom. The resultant dipole moment of $NH_3$ is greater than that of $NF_3$.
Reason $R$: In $NH_3$,the orbital dipole due to the lone pair is in the same direction as the resultant dipole moment of the $N-H$ bonds. $F$ is the most electronegative element.

Although both $CO_2$ and $H_2O$ are triatomic molecules,the shape of $H_2O$ molecule is bent while that of $CO_2$ is linear. Explain this on the basis of dipole moment.

If $HCl$ molecule is completely polarized,the expected value of the dipole moment is $6.12 \ D$,but the experimental value of the dipole moment is $1.03 \ D$. Calculate the percentage ionic character.

Bond distance in $HF$ is $9.17 \times 10^{-11} \ m$. Dipole moment of $HF$ is $6.104 \times 10^{-30} \ Cm$. The percentage ionic character in $HF$ will be : .............. $\%$
(electron charge $= 1.60 \times 10^{-19} \ C$)

The most polar bond is