I am really confused of writing half equations for electrolysis of the molten salts?

Ok well first think of the oxidation numbers of each ion in the molecules.
In (a) Mg is +2 and F is -1.

In electrolysis, we are pumping electrons into the cathode to reduce a substance
Hence the Mg is the one being reduced, it is gaining electrons, to go from a positive ion to a neutral metal

Mg2+ + 2e >>>>> Mg

And the F- needs to lose an electron to become neutral, therefore it is being oxidised at the anode

F- >>> F + e




Try the same with the other salts