The main constituent of a pearl is calcium carbonate. A pearl is formed inside the shell of an oyster or mollusk when an irritant such as a piece of sand or a parasite enters the shell. In response, the mollusk secretes layers of calcium carbonate, also known as nacre, to coat the irritant and form a pearl.

Option 1 is not correct because it includes both calcium carbonate and magnesium carbonate. While calcium carbonate is a main constituent of a pearl, magnesium carbonate is not.

Option 2 is also not correct because it states that the main constituent is calcium sulphate. However, calcium carbonate, not calcium sulphate, forms the majority of a pearl.

Option 3 is not correct because it includes calcium oxide and calcium sulphate. Calcium oxide is not a main constituent of a pearl.

Therefore, the correct answer is option 4, which states that the main constituent of a pearl is calcium carbonate.