Final answer:
The diploid structure in mosses is the capsule.
Step-by-step explanation:
The diploid structure in mosses is called the capsule. The capsule is a spore-producing structure that grows out of the gametophyte stage. It has a stalk called the seta that elevates the capsule for spore dispersal. The capsule contains the diploid sporophyte, which releases spores under favorable conditions.