the answer
the only equation which is balanced in an acidic solution is
D. SO42- + 4H+ + 2e- ------------------ SO2(g) + 2H2O(l)
proof:
it is an acid solution because we can see H+, presence of acid in solution
A. SO42- + 2H2O(l) + 2e- SO2(g) + 4OH-
B. SO42- + 4H+ SO2(g) + 4OH- + 2H2O(l) are basic solution, cause we can observe on these solutions the presence of OH-,
as for SO42- + 2e- --------------------SO2(g) it is not an acid solution
