Answer: b. 0.089
Explanation: The Law of Conservation of Mass states that when in a chemical or physical change, the products' mass of a chemical reaction will equal to the mass of the reactants' mass or in other words, mass cannot be created or destroyed.
The same can be said to moles, since, they are also measures of mass.
In the reaction:
[tex]CH_{3}CN_{(g)} \rightarrow CH_{3}NC_{(g)}[/tex]
initially there were 0.200 mol of reactant. After 25 min, there are 0.111 mol left.
This means that 0.089 mol of reactant "transformed" into product.
Therefore, in the reaction vessel, there are 0.089 mol of product.