Explanation:

- Option 1: Aluminium Oxide

- Often used as an abrasive in toothpaste to remove plaque and stains without scratching tooth enamel.

- Option 2: Silica

- Commonly used as a mild abrasive in toothpaste. It helps polish the teeth and improve their whiteness.

- Option 3: Limestone

- Limestone isn’t typically used directly in toothpaste. However, it may be a source of calcium carbonate, which is common in toothpaste as an abrasive and thickening agent.

- Option 4: Galena

- Galena is a lead ore and not used in toothpaste. It’s primarily used in making lead and is unsuitable for dental products.

- Galena is NOT used in toothpaste.