The classification of fungi is based on various criteria that broadly include morphology, genetics, and physiological features. Each of the provided options, except one, is a valid criterion for the classification of fungi:

• Morphology of mycelium: Fungi can have different types of mycelium, which refers to their vegetative structure. Fungi can be classified based on whether they have septate (with cross-walls) or coenocytic (without cross-walls) mycelium. This morphological characteristic is crucial for classification.
• Mode of spore formation: Spore formation in fungi is a key reproductive strategy and varies significantly among different groups. Spores can be produced sexually or asexually, and the structures involved in spore production are critical for classification. Fungi are often categorized based on whether they form spores inside a sac (ascospores) or on basidia (basidiospores), among others.
• Fruiting body: The presence and type of fruiting body (such as mushrooms, toadstools, puffballs, etc.) are used in fungi classification. These structures are involved in the production of spores and can be quite distinctive among different fungi groups.

However, the option that is not a criterion for the classification of fungi is:

Mode of nutrition: While fungi are heterotrophic (meaning they obtain their food by absorbing dissolved molecules, typically from decomposing material), this characteristic is generally true across all fungi. They can be saprophytic (decomposers), parasitic (feeding on living organisms), or mutualistic (engaging in mutually beneficial relationships). Despite these differences in their ecological roles or interactions, the mode of nutrition is not typically used as a primary criterion for formal taxonomic classification in fungi. It is more pertinent to ecological or functional groupings rather than taxonomic classification.

Therefore, the correct answer is Option B: Mode of nutrition.