What structure, formed due to the conservation of angular momentum, acts as a reservoir for the protostar?

Protostellar disk or accretion disk

As a collapsing core shrinks rapidly, the conservation of angular momentum dictates that its rotation speed must increase dramatically—analogous to an ice skater pulling their arms in. This rapid rotation generates centrifugal force that prevents all the infalling material from striking the center directly. Instead, the material flattens into a pancake-like structure surrounding the central hot core; this structure is the protostellar disk, also known as the accretion disk. This disk is functionally critical because it serves as a reservoir, funneling gas and dust slowly and progressively onto the central protostar over extended periods, thereby allowing the central object to grow in mass.