The part labelled I is the zygospore.

A zygospore in Rhizopus is a thick-walled, resting spore formed through sexual reproduction when two compatible hyphae (male and female) fuse together, creating a structure that contains multiple nuclei which then fuse to form diploid nuclei, essentially acting as the "seed" for a new Rhizopus individual; it's a key characteristic of this fungus and is considered a key identifying feature of the Zygomycete phylum to which Rhizopus belongs.