The coal with the highest ash content is

Classification of coal is done on the basis of carbon content. The least carbon-containing coal will have the highest ash content. The carbon content decreases in the order-
Anthracite> Bituminous> Steam> Coking> Lignite. 

78% to 86% of bituminous coal is made up of carbon. Additionally, it has 12% ash content, 17% moisture, and a small quantity of nitrogen. Additionally, bituminous coal includes several volatile substances, such as sulphur and other hydrocarbons. It is often referred to as soft coal and black coal.
Due to its brown colour, lignite is often referred to as brown coal. It is the kind of coal with a carbon content between 20% and 30%, which is the lowest.
Steam coal has a carbon content of between 77% and 87%. It is dry coal, and steam coal has a heating value that is significantly higher than that of lignite and sub-bituminous coal.
The carbon content of coking coal ranges from 43% to 86%. This sort of bituminous coal exists.
As we can see, lignite has the least amount of carbon, which indicates that it will have the most ash.

Hence, the correct answer is option C.