The correct option is D Inverse of heat capacity of vapour
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.
In the given graph, AB and BC represent the phase changes in solid material. At AB, the solid material converts into liquid and at BC, it converts into vapour.
i.e in part DE, the material is in vapour state.
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.