Answer: $1.67 \times 10^{-4}\;M/s$

For the given reaction, rate of the reaction $ = - \large\frac{1}{3}$$ \large\frac{\Delta B}{\Delta t}$$ =\large\frac{1}{2}$$\large\frac{\Delta C}{\Delta t}$

$\Rightarrow \large\frac{\Delta C}{\Delta t}$$ = \large\frac{2}{3}$$ \large\frac{\Delta B}{\Delta t}$$ = \large\frac{2}{3}$$ \times\; 2.50 \times 10^{–4}\;M/s$$ = 1.67 \times 10^{–4}\;M/s$..