The horizonatally growing hyphae of Rhizopus are called

The correct answer is B. Stolons.

Stolons are horizontal hyphae that grow along the surface of a substrate. They are used for vegetative reproduction and to spread the fungus to new areas.

Rhizoids are thread-like structures that anchor the fungus to the substrate. They are not involved in reproduction.

Sporangiophores are upright hyphae that bear sporangia. Sporangia are structures that produce spores.

Holdfasts are structures that anchor the fungus to a substrate in aquatic environments. They are not found in Rhizopus.