Tooth enamel, the outermost layer of the teeth, is composed mainly of a mineral called hydroxyapatite. Hydroxyapatite is a compound made up of calcium and phosphate ions. This compound is extremely hard and durable, which makes tooth enamel strong and resistant to wear and tear.

Option 1: Calcium carbonate is a compound commonly found in rocks and minerals, but it is not the main component of tooth enamel.

Option 2: Calcium sulphate is a compound that is often used as a food additive or in plaster of Paris, but it is not a component of tooth enamel.

Option 3: Calcium hydroxide is another compound that can be found in various applications, such as in the production of cement or as an antacid. However, it is not a major constituent of tooth enamel.

Option 4: Calcium phosphate is the correct answer. It is a major component of tooth enamel and is responsible for its hardness and strength.

In summary, tooth enamel is primarily composed of hydroxyapatite, a compound made up of calcium and phosphate ions. Calcium phosphate is the correct answer as it is the main calcium compound found in tooth enamel.