# The ________ sphere is enclosed in brackets in formulas for complex species, and it includes the central metal ion plus the coordinated groups.

$(a)\;ligand\qquad(b)\;donor\qquad(c)\;oxidation\qquad(d)\;coordination$

The coordination sphere is enclosed in brackets in formulas for complex species, and it includes the central metal ion plus the coordinated groups.
Hence (d) is the correct answer.