Step 1: Concept
Peat formation in bog ecosystems.
Step 2: Meaning
Peat forms when organic matter accumulates and decomposes very slowly under waterlogged, acidic conditions.
Step 3: Analysis
Sphagnum (peat moss) absorbs large quantities of water (up to 20 times its dry weight), creates and maintains the acidic, anaerobic conditions that inhibit decomposition, and directly contributes its biomass to peat formation. The other mosses (Riccia, Funaria, Polytrichum) do not form significant peat deposits.
Step 4: Conclusion
Sphagnum is the primary agent of peat formation in bog systems.
Final Answer: (D)