Explanation:

x In the Gandhara region, Indian and Hellenistic features coalesced. There was a significant expansion of Buddhist monasteries in Gandhara and northern Afghanistan in the early centuries CE, but very little evidence of their architectural form survives. Takht-i-Bahi in Pakistan and Guldara in Afghanistan are two important sites. x Excavations at Takht-i-Bahi revealed a large monastic complex that included several connected clusters of cells arranged around courtyards, stupas, and sculptures. A stupa once stood in one of the courtyards, but only its square base survives. x In contrast to the stupas of central India, those of the northwest had a towerlike appearance with sculptural decoration on the base and dome. Guldara stupa (2nd century CE) rises from a high square base, with stairs leading up to it from the east. The outer façade of the structure consists of thin, flat slabs of sedimentary rock, carefully arranged one on top of the other (this is known as the diaper masonry technique and was introduced to this region by the Parthians), the interior filled with stone rubble. x Extensive evidence of early Buddhist shrines and stupas comes from Taxila. The city of Sirkap at this site was founded by the Indo-Greeks and continued to be occupied during Shaka and Parthian rule. The excavated remains mostly belong to the latter phase. The largest structure is a ruined Buddhist apsidal temple, located in Block D. This has a screen between the apse and area for assembly. A number of stone heads showing Indian and Greek features and styles were found here; some may represent bodhisattvas. In front of the shrine, on both sides of the entrance, were the square bases of stupas. This structure can be dated to the first half of the 1st century CE. Another important structure at Sirkap was the so-called ‘shrine of the double eagle’ in Block F, probably built in the late 1st century BCE. Its only surviving portion is the square base of what was obviously a stupa. x Hence, option (b) is the correct answer.