2015 AMC 12A Problem 14

By the change-of-base formula, $\dfrac{1}{\log_b a} = \log_a b.$ Therefore $$\begin{aligned} &1 = \log_a 2 + \log_a 3 \\ &\quad {}+ \log_a 4 = \log_a 24. \end{aligned}$$ It follows that $a = 24.$ Thus, the correct answer is D.