The mass or sheet like aggregation of a slime mould (Myxomycetes) is known as

The correct option is B plasmodium
Slime moulds are protists that are saprophytic and are commonly found on decaying plant matter.
True slime moulds are included under the phylum Myxomycetes.
They vegetatively grow and form a sheet-like aggregation called plasmodium under suitable conditions. During unfavourable conditions, the fruiting bodies bearing spores are formed due to differentiation of plasmodium.
Protonema is the first stage in the gametophytic life cycle of mosses. It develops from a spore.
Mycelium is usually referred to the network of hyphae in fungi.