Final answer:
At Sant'Ambrogio, the early Christian architectural form present is a basilica plan with a narthex, which serves as an entrance or lobby at the west end of the nave, distinct from central plan, campanile, or baptistery. The correct answer is option a).
Step-by-step explanation:
The early Christian architectural form that appears at Sant'Ambrogio is a basilica plan with a narthex. This type of church architecture features a central nave with aisles on each side and an apse at one end and often includes a transept to give it a cruciform shape. The narthex serves as an enclosed entrance or lobby area at the west end of the nave, prime for gathering before entering the main body of the church.
While central plan churches, like San Vitale in Ravenna, are characterized by a central space capped with a dome and circular or polygonal in shape, the basilica plan is more longitudinal. Unlike a campanile, which is a freestanding bell tower, or a baptistery, which is a separate building used for baptism, the narthex is an integral part of the basilica's plan and serves as a threshold space.