Given the fact "peat is made of decaying vegetation compacted by soil in a swamp over a long period of time", what is the answer to the question or completion "Peat is made by decaying vegetation compacted by soil in a"

Possible answers:
[a]. uncultivated swamp or marsh;
[b]. river that flows with freshwater;
[c]. geographically separated saltwater ocean;
[d]. lake surrounded by land with a basin;
[a].