Ellingham diagram is a graphical representation of:

The Ellingham diagram is a widely used tool in metallurgy that helps predict the feasibility of reduction of metal oxides. It is a plot of the standard Gibbs free energy of formation ($$\Delta G$$) of oxides per mole of $$O_2$$ consumed versus the absolute temperature ($$T$$).

The relationship $$\Delta G = \Delta H - T\Delta S$$ shows that $$\Delta G$$ varies linearly with temperature (assuming $$\Delta H$$ and $$\Delta S$$ remain approximately constant over a temperature range). The Ellingham diagram plots these linear relationships for various metal-oxide systems, allowing comparison of the relative stabilities of different oxides at any given temperature.

Among the given options, the Ellingham diagram represents $$\Delta G$$ vs $$T$$, which corresponds to option (4).