"X" is an oxoanion of the lightest element of group 7 (in the periodic table). The metal is in +6 oxidation state in "X". The color of the potassium salt of X is

We need to identify the oxoanion X of the lightest element of group 7, with metal in +6 oxidation state.

First, the lightest element of group 7 (Group VIIB / Mn group) is Manganese (Mn).

Next, Mn in +6 oxidation state forms the manganate ion: $$MnO_4^{2-}$$. Check: Mn + 4(-2) = -2, so Mn = +6. ✓

Then, potassium manganate $$K_2MnO_4$$ has a green color (this is different from $$KMnO_4$$ which is purple, and $$K_2Cr_2O_7$$ which is orange).

Therefore, the color is Option 4: green.