Identify the water-gas shift reaction

The water-gas shift reaction is: $\mathrm{CO + H_2O \rightleftharpoons CO_2 + H_2}$

Determine what happens to each species

• Carbon monoxide, $\mathrm{CO}$, changes to carbon dioxide, $\mathrm{CO_2}$.
• In this change, carbon goes from oxidation state: $+2 \text{ in } \mathrm{CO} \to +4 \text{ in } \mathrm{CO_2}$
• Increase in oxidation state means oxidation.

So, carbon monoxide is oxidized to carbon dioxide.

Check each option

A: water is evaporated in presence of catalyst.
This is not the defining chemical change of the water-gas shift reaction. Incorrect.

B: carbon monoxide is oxidized to carbon dioxide.
This matches the reaction exactly. Correct.

C: carbon is oxidized to carbon monoxide.
This refers to formation of water gas, not the water-gas shift reaction. Incorrect.

D: carbon dioxide is reduced to carbon monoxide.
This is the reverse process, not the usual statement of the water-gas shift reaction. Incorrect.

Final answer

The correct option is: $\boxed{\text{B}}$

Comparison with stored answer

Stored correct answer: B
Derived answer: B
Hence, they agree.