The problem requires identifying the set of live variables at the exit point of each basic block in a control flow graph. Let's analyze each block step-by-step:

1. Basic Block B4:
   • The formula for \(g\) is \(g = d + e\).
   • At the exit of B4, no successors need any variables; hence, it's an empty set.
2. Basic Block B3:
   • The formula for \(e\) is \(e = a + f\).
   • At the exit, we check the needs of B2 and B4. B2 uses \(d\) and \(e\), B4 uses \(d\) and \(e\).
   • Thus, variables \(b, c, e, f\) are needed at the exit of B3 to supply B2 and B4 correctly.
3. Basic Block B2:
   • The formula for \(d\) is \(d = a + e\).
   • At the exit of B2, \(d\) and \(e\) must still be live for B4.
4. Basic Block B1:
   • The formula for \(a\) is \(a = b + c\).
   • At the exit, variables \(a, b, c, e, f\) are needed as they are used or defined in subsequent blocks (B2 and B3).

The correct answer is: B1:{a, b, c, e, f}, B2:{d, e}, B3:{b, c, e, f}, B4:\(\emptyset\)