Step 1: Understanding the Concept:
For an ideal gas, \(Z = 1\). The deviation of \(Z\) from 1 shows how far a real gas departs from ideal behaviour.
Step 2: Key Formula or Approach:
\[ Z = \frac{PV}{nRT} \]
Step 3: Calculate.
\(P = 40\) atm, \(V = 0.4\) dm\(^3\), \(n = 1\), \(T = 300\) K, \(R = 0.082\).
\[ PV = 40\times 0.4 = 16 \]
\[ nRT = 0.082\times 300 = 24.6 \]
\[ Z = \frac{16}{24.6} = 0.65 \]
Step 4: Check the options.
\(Z = 1\) would be an ideal gas. Here \(Z < 1\), so attractive forces dominate.
Final Answer:
The compressibility factor is 0.65, option (B).
\[ \boxed{Z = 0.65} \]