The degree of polarity of a covalent bond is given by the dipole moment ($\mu$).

Which of the following chemical species is non-polar?

The three $\mathrm{P}-\mathrm{Cl}$ bonds dipole moments cancel each other out at their equatorial positions, whereas the two $\mathrm{P}-\mathrm{F}$ bonds' dipole moments cancel each other out at their axial positions, giving rise to the zero-dimensional dipole moment of ${\mathrm{PF}}_2{\mathrm{Cl}}_3$.