How does mycelia contribute to the growth of decay fungi?

Mycelia play a crucial role in the growth of decay fungi primarily through their ability to break down cellulose and lignin, which are complex organic materials found in plant cell walls. These components are abundant in dead plant matter, making them essential for fungi that contribute to the decomposition process.

Mycelia are composed of a network of thread-like structures called hyphae, which expand throughout the substrate. This expansive structure allows the fungi to secrete enzymes that can effectively degrade cellulose and lignin, converting them into simpler compounds that the fungi can easily absorb as nutrients. This process not only facilitates the fungi's growth and reproduction but also plays an essential role in nutrient cycling within ecosystems, as it helps to recycle organic materials back into the soil.

The other options do not accurately reflect the primary function of mycelia in decay fungi. For example, fungi do not conduct photosynthesis; they are heterotrophic and rely on breaking down organic material for energy. Likewise, while mycelia may inadvertently help retain moisture in some contexts, this is not their primary function in promoting the growth of decay fungi. Mycelia's direct interaction with organic substrates and their role in the decomposition process underscore the importance of option C as the correct answer.