Which type of fuel was formed by seedless plants that died 300 million years ago?

The type of fuel formed by seedless plants that died around 300 million years ago is coal. During the Carboniferous period, dense forests of seedless plants, primarily ferns and horsetails, accumulated in swampy environments. Over millions of years, the remains of these plants underwent heat and pressure, transforming them into coal, a major fossil fuel source used for energy.