Final answer:
The underground horizontal stem is a rhizome, which is a specialized structure for vegetative asexual reproduction and enables plants like ginger and ferns to spread and grow.
Step-by-step explanation:
The correct answer to which structure is an underground horizontal stem is b. rhizome.
Rhizomes are specialized structures that function in vegetative asexual reproduction. Unlike bulbs or corms that are mainly used for storage, rhizomes grow horizontally underground and permit plants to spread out into new area. These perennial organs have nodes and internodes, and new vertical shoots can arise from the buds on a rhizome. Plant species like ginger and ferns make use of rhizomes to establish new growth in the following season.
Stolons and runners, such as those produced by strawberry plants, may look similar but these are typically aboveground stem adaptations that allow plants to spread horizontally at or just below the soil surface. Corms and bulbs, although also underground structures, are more rounded, fleshy, and are primarily for storage. Tubers like potatoes have 'eyes' which are adventitious buds capable of developing into new plants.