What is A in the following reaction?

The correct sequence of correct reagents for the following transformation is :

The addition of silica during the extraction of copper from its sulphide ore :-

The equilibrium constant for the reaction

A(s) $$\rightleftharpoons$$ M(s) + $${1 \over 2}$$O₂(g)

is K_p = 4. At equilibrium, the partial pressure of O₂ is _________ atm. (Round off to the nearest integer)