In simple interest, interest is calculated on principal amount. Let $S$ be the principal amount.
$$\begin{equation} S \times r \times 2=2400\label{ref1} \end{equation}.$$
The simple interest for 1 year is 2400/2 = 1200.
For compound interest, interest is paid on this interest also and the difference of SI and CI for 2 years will be exactly this interest for 1200.
So, $$\begin{equation} 1200 \times r = 2520 - 2400 = 120 \\ \implies r = 0.1\label{ref2}\end{equation}.$$
From $\ref{ref1}$ and $\ref{ref2}$
$S = \frac{2400}{2 \times 0.1} = 12000$.
So, a choice.