The correct answer is option 3: thinner.

Turpentine oil is commonly used as a solvent and thinner in oil-based paints. When added to paint, it helps to dilute the paint, making it easier to apply and spread on surfaces. It also helps to control the viscosity or thickness of the paint, allowing for better flow and leveling. Turpentine oil acts as a solvent, breaking down the binders and pigments in the paint, which helps the paint to dry faster and more evenly.

Option 1: Pigment is not the correct answer. Pigments are substances that give paint its color but turpentine oil is not used for that purpose.

Option 2: Film-forming material is not the correct answer. Film-forming materials are binders or resins that help the paint to adhere to surfaces and form a durable film. Turpentine oil is not used for this purpose.

Option 4: Drier is not the correct answer. Driers are additives that are used to accelerate the drying time of paint. Turpentine oil does not have drying properties and is not used as a drier.