What is a combustible black or dark brown rock mainly composed of carbonized plant matter?

The substance described as a combustible black or dark brown rock mainly composed of carbonized plant matter is indeed coal. Coal is formed from the remains of ancient plant life that has been subjected to heat and pressure over millions of years, leading to the carbonization process. This high carbon content is what makes coal an efficient fuel source, capable of producing significant amounts of heat and energy when burned.

The other options represent different forms of fossil fuels or carbon-rich materials. Petroleum and natural gas are liquid and gaseous hydrocarbons, respectively, formed from the remains of marine organisms, not plant matter. Charcoal, although a form of carbon derived from burning wood in a low-oxygen environment, is not classified as a rock but rather as a material used for various purposes including fuel. Therefore, the defining characteristics of coal align precisely with the description provided, making it the correct answer.