Out of the following which form of coal has the highest content of carbon? 

Peat is an organic deposit. Burial, compression, and carbonization turn it into coal and stone. The carbon content on a dry ash-free base is less than 60%. 

Lignite is the lowest type of coal. It is peat turned into a rock, which is a mass of brown charcoal. Lignite may contain recognizable plant structures. It has a carbon content of 60-70% on a dry ashless base. 

Sub-bituminous coal is lignite with increased organic metamorphism. This metamorphosis expelled some of the oxygen and hydrogen from coal. This loss produces chars with a high carbon content (71-77% on a dry ash-free basis). 

Bitumen is the most common type of coal. Bituminous coals are formed when sub-bituminous coals are exposed to increased levels of organic metamorphism. It has a carbon content of 77-87% on a dry ashless base.

Anthracite is the highest rank of coal. Unlike other types of coal, it is usually considered metamorphic rock. It has a carbon content of  87% or more on a dry ash-free base.

Therefore, the correct answer to this question is option A.