To solve this problem, we need to identify which organism is responsible for peat formation.$\newline$Let's analyze each option:$\newline \bullet$Marchantia:$\newline$ - Marchantia is a genus of liverworts, which are non-vascular plants.$\newline$ - They do not contribute significantly to peat formation.$\newline \bullet$Riccia:$\newline$ - Riccia is another genus of liverworts.$\newline$ - Like Marchantia, they are not major contributors to peat formation.$\newline \bullet$Funaria:$\newline$ - Funaria is a genus of mosses, commonly known as cord moss.$\newline$ - While mosses can contribute to organic matter in soil, Funaria is not primarily responsible for peat formation.$\newline \bullet$Sphagnum:$\newline$ - Sphagnum, commonly known as peat moss, is a genus of mosses that plays a crucial role in peat formation.$\newline$ - Sphagnum mosses accumulate in bogs and wetlands, where they decompose slowly, leading to the formation of peat.$\newline$ - Peat is an accumulation of partially decayed vegetation or organic matter, primarily formed in wetland conditions.$\newline$Therefore, the organism responsible for peat formation is Sphagnum.$\newline$This corresponds to Option 4, which is the correct answer.