The coverage for this structure is θ = 1/2  (θ = 0.5)

Rationale

The coverage can be determined simply by counting atoms within a given area of surface, but the area chosen must be representative of the surface as a whole.

If we take the adsorbate unit cell then

θ = (No. of adsorbate species/ No. of substrate atoms) = (4 x ¼ + 1) / (4 x ¼ + 4 x ½ + 1) = 2/4

i.e.  θ = 1/2

The coverage can also be obtained by comparing the areas of the unit cells of the adsorbate and substrate, with additional allowance for the number of atoms per unit cell.

In this instance

1. the primitive unit cell of the substrate contains one surface substrate atom - call this the unit area.
2. the c(2x2) unit cell contains 2 adsorbate atoms and has a relative area of 4 units.

Hence,      θ  = (1/4) x 2 = 1/2