Which molecule is polar?

Which molecule has a zero dipole moment?

Given below are two statements:
Statement-$I$: Since fluorine is more electronegative than nitrogen,the net dipole moment of $NF_3$ is greater than $NH_3$.
Statement-$II$: In $NH_3$,the orbital dipole due to lone pair and the dipole moment of $N-H$ bonds are in opposite direction,but in $NF_3$ the orbital dipole due to lone pair and dipole moments of $N-F$ bonds are in same direction.
In the light of the above statements,choose the most appropriate from the options given below.

Which one of the following molecules is polar?

$(a)$ Discuss the significance/applications of dipole moment.
$(b)$ Represent diagrammatically the bond moments and the resultant dipole moment in $CO_2, NF_3$ and $CHCl_3$.

Arrange the following compounds in increasing order of dipole moment:
$CH_3CH_2CH_3$,$CH_3CH_2NH_2$,$CH_3CH_2OH$