The molecules ${\mathrm{BF}}_3$ and ${\mathrm{NF}}_3$ are covalent compounds. But ${\mathrm{BF}}_3$ is non-polar and ${\mathrm{NF}}_3$ is polar. The reason is

It is given to find out the reason why  ${\mathrm{BF}}_3$ is non-polar and ${\mathrm{NF}}_3$ is polar. 

Explanation
${\mathrm{BF}}_3$ is a triangular planar molecule whereas ${\mathrm{NF}}_3$ has pyramidal geometry.

The molecule ${\mathrm{BF}}_3$ and ${\mathrm{NF}}_3$, both are covalent compounds but ${\mathrm{BF}}_3$ is non-polar and ${\mathrm{NF}}_3$ is polar. The reason is that ${\mathrm{BF}}_3$ is a symmetrical molecule whereas ${\mathrm{NF}}_3$ is unsymmetrical.
In a symmetrical molecule, the individual bond dipoles cancel each other whereas, in unsymmetrical molecules, they do not.
${\mathrm{BF}}_3$ is a triangular planar molecule whereas ${\mathrm{NF}}_3$ has pyramidal geometry.

So, option D is correct.