Question

Please help me out with this.

Answer 1

It appears to me that the first reaction is the correct answer.
   
              N2 O4 (g)  <===>  2 NO2 (g)
initial      x  moles                 0 moles
equilibrium   x (1-α)              2 x α moles

Total number of moles at equilibrium: x (1+α) 
Partial Pressure of NO2:  2 x α/[x(1+α)]  * P = 2 α P /(1+α) .
                         of N2O4 :   x(1-α)/x(1+α)  * P = (1-α) P /(1+α)

Equilibrium constant Kp in terms of partial pressures.
   Kp = [ 2αP/(1+α)]² / [(1-α)P/(1+α) ]
         = 4 α² P / (1 - α²)


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Equilibrium constant Kp of other reactions are different.
2)  2 H I (g)  <===> H2 (g) + I2 (g)

        Kp = c * α² /(1-α)²,    c = constant

3)  2 NH3 (g)   <===>  N2 (g) + 3 H2 (g)
                      Kp = c * α⁴ P² /(1 - α²)²

4)  P Cl5 (g)  <==== >  P Cl3 (g) + Cl2 (g)

       Here Kp = α² P /(1 - α²)