What is the primary safety issue in the block-building center?

The primary safety issue in the block-building center relates to the height of the building structures. As children stack blocks to create taller structures, there is an increasing risk of those structures toppling over. These toppled buildings can pose a danger to children nearby, as falling blocks can cause injuries.

Additionally, as children become more ambitious in their building efforts, the height can lead to situations where they might climb on the structures, further increasing the risk of falls. Ensuring that children build within safe height limits can help mitigate these risks and create a safer play environment.

In comparing the other options, while the type of blocks and their age appropriateness are important considerations for ensuring that materials are safe and suitable for children, they do not directly address the immediate and dynamic safety concerns presented by the height of the structures being built. Cleanliness of the area is important for general safety and health but is not the most pressing concern within the context of block-building activities, where height directly influences safety risk.