A group VII element which has a +7 oxidation state forms a salt with potassium (K). What is the colour of this salt?

The correct option is Purple.

Let us break down the logical reasoning step-by-step to understand how we arrive at this answer:

1. Identify the Element:
The question refers to a Group VII (Group 17) halogen element that has a +7 oxidation state. In the periodic table, manganese (Mn) is a transition metal that commonly exhibits a +7 oxidation state (in permanganates), but among the halogens, chlorine (Cl), bromine (Br), and iodine (I) can form oxyanions where the halogen has a +7 oxidation state (known as perhalates, such as perchlorate, perbromate, and periodate). However, when we consider common salts of manganese in its +7 oxidation state (which behaves like a Group VII transition element in older periodic table designations), or potassium permanganate (KMnO₄), the manganese is in the +7 oxidation state.
Specifically, potassium permanganate (KMnO₄) contains potassium (K⁺) and the permanganate ion (MnO₄^-), where manganese (Mn) is in the +7 oxidation state:
$\mathrm{oxidation\; state\; of\; Mn}+4(-2)=-1\Rightarrow \mathrm{oxidation\; state\; of\; Mn}=+7$
Potassium permanganate is a well-known salt formed with potassium (K).

2. Determine the Color of the Salt:
Potassium permanganate (KMnO₄) is famous for its intense dark purple (or violet) color in solid form and deep purple color when dissolved in water. This characteristic color is due to a charge-transfer transition from the oxygen ligands to the manganese center (ligand-to-metal charge transfer).

Therefore, the color of the salt is purple.