Which of the following cannot act as reducing agent? 

In all the given compounds oxidation number of non metal is + 4. As C belongs to group IV and it is in its maximum oxidation state. So, reduction in oxidation number of nonmetal is not possible only in CO₂,. As we know that reduction is always accompained by an increase in 
oxidation number of reducing agent. So, CO₂ cannot acts as reducing agent among the given choices.