What is the name of the long, thin filament that composes the body of a fungus?

The correct answer is hyphae, which refers to the long, thin filaments that make up the structure of fungi. These filamentous structures are critical for the growth and nutrient absorption processes of fungi. Hyphae can extend over large areas and are the main components of the fungal body, known collectively as mycelium when they form a network. This network increases the surface area available for absorbing nutrients from the surrounding environment, playing a vital role in the fungus's ability to thrive in various habitats.

Hyphae can be categorized into two types: septate hyphae, which have cross-walls (septa) separating the cells, and coenocytic hyphae, which lack septa and consist of a continuous cytoplasmic mass with multiple nuclei. This structural diversity allows fungi to adapt to different ecological niches.

In contrast, while mycelium represents the entire mass of hyphae, thallus refers to the overall body structure of a fungus, encompassing mycelium and reproductive structures, but not specifically the filament itself. Stolon refers to a horizontal branch that grows from the base of a plant, which does not apply to the fungal structure in question.