6. According to Crystal Field Theory, what should be the color of an aqueous solution of the ion [Y(H2O)6]3

Colourless

Explanation:

We know that Y^3+ has the electronic configuration of;

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 (the 5s and 4d levels are empty).

According to the crystal field theory, the colour of complexes result from transitions between incompletely filled d orbitals.

As a result of this, complexes with empty or completely filled d orbitals are colourless. Thus,  [Y(H2O)6]3 is colourless according to the Crystal Field Theory.

it is the most commonly accepted one