Final answer:
CCl4, with its larger size, has stronger London dispersion forces, resulting in a higher melting point and lower vapor pressure compared to the smaller CS2 molecule.
Step-by-step explanation:
The melting point and vapor pressure of a substance are influenced by the intermolecular forces present between its molecules. For nonpolar molecules such as carbon disulfide (CS2) and carbon tetrachloride (CCl4), the only intermolecular forces operating are London dispersion forces. Since CCl4 is a larger molecule than CS2, it has more electrons and a larger surface area, which leads to stronger London dispersion forces and subsequently, a higher melting point and lower vapor pressure when compared to CS2.