**Anthracite coal** produces the most heat per unit among the given types of coal.

**Explanation:**

Coal is a combustible black or brownish-black sedimentary rock that is primarily composed of carbon, along with various other elements such as hydrogen, sulfur, oxygen, and nitrogen. It is formed from the remains of plants that lived and died millions of years ago.

There are several types of coal, classified based on their carbon content, energy content, and heat-producing ability. The types of coal listed in the question are:

a) Coal: This is a general term used to refer to all types of coal. It includes various types such as lignite, bituminous coal, and anthracite.

b) Lignite: Lignite coal, also known as brown coal, is the lowest rank of coal. It has a relatively low carbon content and a lower heat-producing ability compared to other types of coal.

c) Anthracite: Anthracite coal is the highest rank of coal. It has a high carbon content and a high energy content, making it the most efficient type of coal in terms of heat production.

d) Pit: "Pit" is not a specific type of coal. It is a term used to refer to a coal mine or an excavation where coal is extracted.

Among these types of coal, anthracite coal is known for its high carbon content, low moisture content, and high energy density. It burns with a clean flame and produces a significant amount of heat per unit. It is often used as a fuel in residential heating, industrial processes, and power generation.

The heat-producing ability of coal is measured in British thermal units (BTUs) per pound. Anthracite coal typically has a heat content of around 24 million BTUs per ton, whereas lignite coal has a heat content of about 4 million BTUs per ton. This difference in heat content indicates that anthracite coal produces significantly more heat per unit compared to lignite coal.

Therefore, the correct answer is option 'C' - Anthracite coal produces the most heat per unit among the given types of coal.