What are the visible structures of fungi commonly observed?

The visible structures of fungi that are commonly observed include characteristic conks and rusts. Conks are definitive structures formed by certain types of fungi, notably the wood-decaying fungi and some others, which have a noticeable fruiting body often found on trees and logs. Rusts refer to a group of fungal pathogens that typically appear as reddish-brown spots on the surfaces of plants, and are significant in agricultural contexts due to their impact on crops.

The other options, while they contain some elements related to fungi, do not specifically highlight the most recognizable or unique structures associated with fungal organisms. For instance, mildews and leaf spots refer broadly to plant diseases caused by various pathogens, not exclusively fungi. Similarly, roots and root hairs pertain to plant biology rather than fungi. Conidia and hyphae, while crucial to the fungal life cycle, are microscopic components that may not always be easily observed without magnification, thus making the conks and rusts more suitable representatives of what is visibly identifiable in various environments.