A,B,C,D,E,F in the given diagram are:

A,B,C,D,E,F in the given diagram are: A- Melting, B- Vapourisation, C- Condensation, D- Freezing, E-Sublimation, F-Deposition.
In A, the solid is being changed into a liquid. Hence, that process is melting. 

In B, the liquid is transitioning into gas and hence B is vapourisation. 

In C, gas is changing into a liquid which is called condensation. 

In D, a solid is being formed from a liquid and hence is known as freezing.

 E and F are sublimation and deposition as a solid is changing into a gas directly and a gas is changing into a solid, respectively.