During the stage of a plant's life cycle where it is a haploid structure, the body of the plant is correctly referred to as a(n)

The haploid structure of the plant, during a stage in plant's life cycle, is known as the gametophyte.

Explanation:

The life cycle of plants includes a haploid generation and a diploid generation. Both the structures are multicellular.

The diploid structure of a plant is known as sporophyte. This structure produces spores via meiotic division.

Whereas, the haploid structure of the plant, known as the gametophyte, is formed from spore and produces haploid gametes.