${\mathrm{SnCl}}_4$ is a covalent liquid because:

${\mathrm{Sn}}^{4+}$ has a high polarising power due to its high charge density, which results in greater anion polarisation and, consequently, greater distortion of the electron clouds of the ${\mathrm{Cl}}^{-}$ ions. Therefore, the most covalent compound is ${\mathrm{SnCl}}_4$.
Hence, the option B is correct.