Which of the following substances does not possess a dipole moment?

Which of the following molecules has the largest dipole moment?

Which of the following molecules has the most polar covalent bond?

Which of the following compounds has the highest 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.

Amongst $BeF_{2}$,$BF_{3}$,$H_{2}O$,$NH_{3}$,$CCl_{4}$ and $HCl$,the number of molecules with non-zero net dipole moment is ...... .