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

One

The answer is (2) One. EDTA is a hexadentate ligand, which means it can donate 6 electron pairs to the central atom. This is enough to form an octahedral complex with a Ca²⁺ ion, which has a coordination number of 6. So, only one EDTA molecule is required.

The other options are incorrect. Three EDTA molecules would not be enough to form an octahedral complex, since the central atom would only have 9 electron pairs. Two EDTA molecules would not be enough either, since the central atom would only have 12 electron pairs. Six EDTA molecules would be too many, since the central atom would have 18 electron pairs, which is too stable.