What cannot be used in place of posts if there is adequate infill between them?

Balusters are vertical posts or supports that are typically used to form a railing system. They are designed primarily for safety and aesthetics, but they do not have the structural capacity needed to replace posts in a framing system. In a railing system, balusters support the top rail and create a barrier, but they cannot take on the load-bearing responsibilities of posts, which are foundational elements that provide necessary strength and stability.

In contrast, railing, pillars, and beams serve different functions in construction: railings are often used in conjunction with balusters, pillars can provide vertical support and can sometimes be used instead of posts depending on their design, and beams are horizontal structural elements that can span openings and support loads, but none of them fully substitute for the specific structural role of posts when there is adequate infill between them.

Therefore, balusters, which lack the necessary load-bearing capacity to act as posts, are correctly identified as the option that cannot fulfill that role of structural support in a framework.