In which case C-C bond length will be highest?

In ${\mathrm{FCH}}_2-{\mathrm{CH}}_2\mathrm{F}$ case  C-C bond length will be highest.

Greater electronegative substituents are bonded to hybrid orbitals with more p-character in accordance with Bent's Rule.

Thus, the bond length will be longer as the p-characters is more.

Electronegative element $\uparrow$, the p-character to attached orbital $\uparrow$$\mathrm{\%} \mathrm{s}$ of $\mathrm{C}-\mathrm{C}$ $\uparrow$, B.L. $\downarrow .$

Hence, the option C is correct.