For the reaction$2\text{A}+\text{B}\to 3\text{C}+\text{D}$ Which of the following does not express the reaction rate?

For the given reaction, the rate of reaction is expressed as $-\frac{1}{2}\frac{d[\text{A}]}{dt}=-\frac{d[\text{B}]}{dt}=+\frac{1}{3}\frac{d[\text{C}]}{dt}=+\frac{d[\text{D}]}{dt}$. Since C is a product, its concentration increases over time, so the sign must be positive. Therefore, $-\frac{d[\text{C}]}{3dt}$ is incorrect.