Which of the following materials are highly flammable?

The correct answer highlights materials that are particularly susceptible to combustion. Hay, straw, and shavings are all organic materials that contain a high amount of combustible fibers. When dried, these items can ignite easily and sustain a fire due to their lightweight and low moisture content. This flammability is a crucial consideration in settings like barns or stables, where large quantities of these materials are stored and can present a significant fire hazard.

In contrast, materials such as wood and metal, while wood is flammable, metal does not burn. Glass and stone are inherently non-flammable materials that do not contribute to fire; they do not ignite or sustain flames. Plastic and rubber can also be flammable, but they do not present the same level of fire risk when compared directly to the combination of hay, straw, and shavings, particularly in environments where these materials are prevalent. Therefore, the specific flammability characteristics of hay, straw, and shavings make this option the most accurate representation of highly flammable materials.