1. Stupas became larger and more decorative in the post-Maurya period:
- The Mauryan Empire, under the rule of Emperor Ashoka, witnessed the construction of numerous stupas. However, it was in the post-Mauryan period that stupas underwent significant changes.
- Stupas are Buddhist structures that contain relics of the Buddha or other important Buddhist figures. They are typically dome-shaped and symbolize the enlightened mind of the Buddha.
- In the post-Mauryan period, stupas started to become larger and more elaborate in design. This can be observed in the architectural style of the Great Stupa at Sanchi, which was constructed during this period. The size of the stupas increased, and they were adorned with intricate carvings, sculptures, and decorative elements.
- The increased size and decorative nature of the stupas in the post-Mauryan period reflected the growing popularity and influence of Buddhism during that time.

2. Wood was the principal building material:
- The construction of stupas in the post-Mauryan period primarily involved the use of stone and brick. Wood was not the principal building material for stupas during this period.
- Stone and brick were preferred due to their durability and ability to withstand the test of time. Wood, being a perishable material, would have been less suitable for the construction of long-lasting structures.
- However, it is worth noting that wood may have been used for temporary structures or elements within the stupa, such as scaffolding during construction or decorative elements such as doors and railings. But overall, stone and brick were the primary materials used in stupa construction during this period.

3. Their torans were evidence of Hellenistic influence:
- Torans are decorative gateways or arches that are often found in front of stupas. They serve as entrances to the stupa and are decorated with intricate carvings and sculptures.
- The presence of torans in the post-Mauryan period can indeed be seen as evidence of Hellenistic influence. Hellenistic art and architecture, which were influenced by the Greek civilization, had a significant impact on the art and culture of the region during this period.
- The design and ornamentation of torans often incorporated elements such as floral motifs, human figures, and architectural elements that were influenced by the Hellenistic style.
- The use of torans in the post-Mauryan stupas reflects the assimilation and synthesis of various cultural influences, including Hellenistic, in the development of Buddhist art and architecture during this period.

Therefore, statement 1 and 3 are correct, while statement 2 is incorrect. The correct answer is option 'C' (1 and 3 only).