Right choice is (c) Adjustable type
The best I can explain: A carburettor mixes fuel and air for internal combustion engines in the proper air–fuel ratio for combustion. To carburet or carburet means to mix the air and fuel or to equip with a carburettor for that purpose. Carburettors have largely been supplanted in the automotive and, to a lesser extent, aviation industries by fuel injection. They are still common on small engines for lawn mowers, rototillers and other equipment.