Q.Out of C and CO, which is a better reducing agent for ZnO ?

Below ~983 K, CO would be the stronger reducing agent (Exercise 5); but ZnO's reduction is carried out at ~1673 K, well above this crossover, where the C,CO line lies below the CO,CO$_2$ line, making carbon the more effective/thermodynamically favoured reducing agent — consistent with the industrial use of coke, $ZnO + C \rightarrow Zn + CO$. …

Step 1 – Recall the temperature-dependence established earlier

As shown for the general C/CO system (Exercise 5), CO is the better reducing agent only below about 983 K; above that temperature carbon itself becomes the better (thermodynamically more favourable) reducing agent, since the C,CO Ellingham line keeps falling with T (steep negative slope) while the CO,CO$_2$ line rises.

Step 2 – The operating temperature for ZnO reduction

Industrially, ZnO is reduced at around 1673 K — a temperature far above the ~983 K crossover.

Step 3 – Conclusion for ZnO specifically

At 1673 K, we are firmly in the temperature region where carbon is the thermodynamically superior reducing agent (its Ellingham line lies below both CO's line and the Zn,ZnO line in this range). This is exactly why zinc oxide is industrially reduced using coke (carbon) directly rather than by carbon monoxide: …

Since ZnO reduction happens at ~1673 K — well above the ~983 K C/CO crossover — apply the established rule (carbon is the better reduci …

• Giving the opposite answer (CO) by not accounting for the actual high temperature at which ZnO is industrially reduced. …