Barium occurs mainly in the ore(s):

Correct answer: (b) Both barite and witherite

Explanation

The two main ores of barium are:

• barite (baryte), barium sulphate (BaSO₄), the most important; it is used in drilling mud for oil wells, in paints and in the "barium meal" for X-rays;
• witherite, barium carbonate (BaCO₃), less common.