mam, could you explain why the answer can't be 1, 2 and 3?

Mkae the RICE table to see the final answer.
Since the question says equal numbers of moles of HCl and O₂  are taken initially, we can simply assume their initial concentrations are equal.

Let the initial concentration of each reactant be 1.
                        R  4HCl(g) +  O2�(g)⇌ 2Cl2�(g)  +  2H2�O(g)
                                       I     1          1       0           0
                                       C     -4x         -x      +2x        +2x
                                       E      1-4x        1-x     2x        2x
From this its clear that II and III are correct.However I can not be predicted as it depends on the value of x.
So this is not something that must always be true.