A piece of ? can be placed between the load and the head of a jack to help prevent a load from slipping off the jack.

Using wood as a piece placed between the load and the head of a jack is effective because of its natural friction properties, which help grip the load securely. Wood has a rough texture that increases friction, minimizing the risk of the load slipping off when it is lifted or supported by the jack. Additionally, wood can conform slightly to the surfaces it contacts, creating a better grip.

While other materials like aluminum, steel, or plastic may be used in different applications, they generally do not provide the same level of frictional resistance as wood. Aluminum and steel, being smooth and harder materials, could potentially allow loads to slip if not properly secured. Plastic can offer some grip but may lack the durability and rigidity that wood can provide in this specific context. Thus, wood is the preferred choice for enhancing safety and stability when using jacks.