The strongest $\mathrm{P}-\mathrm{O}$ bond is found in the molecule :

Maximum % s-character in $\mathrm{P}-\mathrm{O}$ bond is for ${\mathrm{F}}_3\mathrm{PO}$ because $P-F$ bond has maximum s-character.

So, the correct answer is ${\mathrm{F}}_3\mathrm{PO}$.
Hence, the correct answer is A.