Step 1: Count protons first
Mg has $12$ protons, Al $13$, Be $4$, C $6$. Each mass number counts protons plus neutrons.
Step 2: Subtract
Mg: $24\to12$ neutrons. Al: $27\to14$. Be: $9\to5$. C: $12\to6$.
So (A)-(II), (B)-(IV), (C)-(I), (D)-(III).
Step 3: Compare
Only option (B) lists exactly this set.
Final Answer:
Neutron counts $12,14,5,6$ give option (B).
\[ \boxed{\text{(B)}} \]