In determining the product that may be produced is to use the principle of the law of conservation of mass where the number of atoms/elements in the reaction either before the reaction (reactants) or after the reaction (products) is the same so that a balanced chemical equation is formed.
In the above reaction, there are two elements that play a role in the reaction, namely H and O
So that the resulting product also has these 2 elements, so we can ignore options A and C because there are elements C and Cl, while choice B does not contain the element H
Thus, the law of mass action in terms of partial pressures is:
Hence, given the partial pressures of phosphorous penta and trichloride and the Kpa, one computes the equilibrium partial pressure of chlorine as shown below: