The correct option is B in AlF3, Al3+ interacts very strongly with the neighbouring F− ions to give a three dimensional structure but in SiF4 no such interaction is possible
Aluminum fluoride is ionic and has high melting point. Large aluminum cation has vacant d orbitals. It attains a coordination number of six towards the relatively small fluorine atom. Due to this, a three dimensional structure is obtained. This is not possible in silicon tetrafluoride. Hence, the melting point of silicon tetra fluoride is much lower than the melting point of aluminum trifluoride.