The correct option is
B Solid fossil fuel
Lignite, also known as brown coal, is a combustible sedimentary rock which is formed from naturally compressed peat. It has a low heat content and used as a fuel to generate energy. Since, it is non-renewable source of energy, it is considered as a fossil fuel and because it's a rock, so it is a solid fossil fuel.
So, the correct answer is 'Solid fossil fuel'.