The correct option is D Inverse of heat capacity of vapour
In the given graph, AB and CD represent the phase changes in solid material. At AB, the solid material converts into liquid and at CD, it converts into vapour.
i.e in part DE, the material is in vapour state.
Let us assume c to be heat capacity of vapour.
We know from the definition of heat capacity that,
dQ=cdT
∴dTdQ=1c = Slope of T−Q graph.
where Q is the heat input and T is the temperature.
Hence, we can conclude that the slope of T−Q graph at DE represents the inverse of the heat capacity of vapour.
Thus, option (d) is the correct answer.