How many EDTA (ethylenediaminetetraacetic acid) molecules are required to make an octahedral complex with a Ca²⁺ ion?

One

The correct answer is option 2.  One.

EDTA (ethylenediaminetetraacetic acid) is a hexadentate ligand, meaning it can form up to six coordinate bonds with a single metal ion. In forming a complex, EDTA typically coordinates through its two amine nitrogen atoms and four carboxylate oxygen atoms.

A Ca\(^{2+}\) ion requires six coordination sites to form a stable octahedral complex. Since EDTA can provide all six coordination sites by itself, only one EDTA molecule is required to make an octahedral complex with a Ca\(^{2+}\) ion.

Therefore, the correct answer is: One.