Step 1: Use the mean-mode gap version of the empirical relation.
The same relation can be written as $\text{Mean}-\text{Mode}=3(\text{Mean}-\text{Median})$.
Step 2: Plug in the known values.
Mean $=43$, Median $=40$, so Mean $-$ Median $=3$, giving Mean $-$ Mode $=3\times3=9$.
Step 3: Solve for the Mode.
$43-\text{Mode}=9 \implies \text{Mode}=34$, matching option (A).
\[ \boxed{34} \]