On an Ellingham diagram of $\Delta G^{\circ }$ against $T$, which of the following lines has a negative slope?

The slope equals $-\Delta S^{\circ }$. Forming 2 mol of CO gas from 1 mol of $\mathrm{O}{}_2$ increases the amount of gas, so $\Delta S^{\circ }>0$ and the slope is negative. For $\mathrm{C}+\mathrm{O}{}_2\to \mathrm{CO}{}_2$ the gas amount is unchanged (slope nearly zero), and the metal oxide lines slope upwards because gas is consumed.