identify the correct sequence in increasing order of the carbon content per unit mass of the following reservoirs of carbon in terrestrial environment

A. Plants

B. Peat

C. Soil organic matter

D. Fossil fuels

Choose the correct answer from the options given below

The Carbon Cycle:

• Carbon is stored in four main reservoirs - oceans (the largest reservoir), followed by geological reserves of fossil fuels, the terrestrial surface (soil and plants mainly), and the atmosphere.
• Natural processes result in a continuous exchange of carbon between the atmosphere, oceans, and terrestrial surface, that is approximately balanced.
• Fossil fuel use and deforestation represent unbalanced additions to atmospheric carbon dioxide.
• Only about half of the anthropogenic carbon dioxide in the atmosphere has been removed and returned to oceans and terrestrial surface by natural processes.
• If continuously more carbon dioxide is emitted into the atmosphere than is being removed, atmospheric concentrations of carbon dioxide will continue to increase. 

Peat forms when plant material does not fully decay in acidic and anaerobic conditions. It is composed mainly of wetland vegetation: principally bog plants including mosses, sedges, and shrubs, etc. Thus it contains less carbon content per unit mass as compared to plants.

Therefore, the increasing order of the carbon content per unit mass of the given reservoirs of carbon in the terrestrial environment is peat, plants, soil organic matter, and fossil fuels.