Option d is the correct answer. The Ashokan stone pillars are not parts of architectural structures. Instead, they are standalone monumental columns erected by Emperor Ashoka across his empire. These pillars often have inscriptions carved onto them, spreading Ashoka's messages and teachings. The pillars are highly polished, monolithic (carved from a single piece of stone), and have a tapering shape with a capital on top. Important Tips Differences between Mauryan pillars and Achaemenian pillars: Material: Mauryan pillars are made from a single piece of stone, while Achaemenian pillars are made up of multiple pieces of stone. Shape: The shaft of a Mauryan pillar is smooth, while the shaft of an Achaemenian pillars has grooves. Capital: The capital of a Mauryan pillar is bell-shaped, while the capital of an Achaemenian pillars is not. Decoration: Mauryan pillars are often inscribed with royal edicts, while Achaemenian pillars are not. Function: Mauryan pillars are free-standing monuments, while Achaemenian pillars are often part of a larger structure.