Biogas mainly consists of:
a. Ethane
b. $CO_2$
c. $H_2$
d. Methane

Hint: Biogas is a mixture of gases. These gases are produced by the methanogenic bacteria which shows the process in the absence of oxygen (anaerobically). This process takes place in anaerobic conditions. Biogas can be produced from organic wastes. Organic wastes such as agricultural waste, manure, municipal waste, plant material, sewage, green waste, or food waste are used.

Complete answer:
> Biogas is composed mostly of methane gas consisting of carbon and hydrogen elements. Methane $(CH_4)$ is the energy compound that is the same as found in natural gas. The composition of biogas varies from 50 to 70% methane with small amounts of water vapor and other gases also. So the correct option is D Methane.

> Carbon dioxide is also the major gas that is found in biogas. It is also an important gas that constitutes 20-25% of the total gas found in the biogas. These are produced from organic waste products. So the option B Carbon dioxide is incorrect.

> Biogas is produced by the methanogenic bacteria anatomically. Biogas consists of many gases. The composition of hydrogen ranges from 0-1%. So option C hydrogen is incorrect.

> Biogas is a renewable source of energy. It is the most efficient method. The other gas composition in biogas is 0.5% which is very less. So option A ethane is incorrect.

Hence, the correct answer is option (D).

Note: Biogas is formed by the decomposition of organic waste in oxygen-free conditions that are anaerobic. It is a mixture of methane and carbon dioxide. Methane is a valuable form of gas, as it is a renewable and efficient source. The amount of $CO_2$ that is produced corresponds to the amount of $CO_2$ captured when the biomass was created.