Final answer:
The circular molding supporting the abacus of a Doric capital is the echinus, which is a defining feature of the Doric order characterized by a simple abacus and a flared echinus. Option a. is the correct answer.
Step-by-step explanation:
The prominent circular molding supporting the abacus of a Doric or Tuscan capital is known as the echinus. In Doric order architecture, the capitals of the columns feature a simple, unadorned square abacus and a flared echinus. The Doric order is the earliest of the Classical orders, characterized by sturdy proportions and the absence of a base. The Doric entablature typically includes a frieze composed of triglyphs and metopes. The echinus plays a functional and aesthetic role, often having a flattened profile that flares into the abacus to support the entablature above.