Step 1: Understanding the Concept
For compression at constant external pressure, the work done on the gas is \(w=P_{\text{ext}}\,(V_{1}-V_{2})\).
Step 2: Volume change
\(V_{1}-V_{2}=10.8-5.4=5.4\ \text{dm}^{3}=5.4\times10^{-3}\ \text{m}^{3}\), and \(w=2460\) J.
Step 3: Calculation
\(P_{\text{ext}}=\dfrac{2460}{5.4\times10^{-3}}=4.556\times10^{5}\ \text{Pa}\approx4.56\) bar, since \(1\ \text{bar}=10^{5}\) Pa.
Final Answer:
The external pressure required is about 4.56 bar.
\[ \boxed{\text{(D) }4.56\ \text{bar}} \]