Which type of slab should never be drilled into?

A post tension slab should never be drilled into due to the high tension cables embedded within it. These cables are critical for the integrity and strength of the slab, as they are under tension and help to manage stress and cracking. Drilling into a post tension slab can potentially damage these cables, leading to structural failure, safety hazards, and costly repairs.

In contrast, reinforced concrete slabs, floating slabs, and slabs on grade do not have the same inherent risks associated with drilling. Reinforced concrete slabs are designed with steel reinforcement to provide strength but can be safely drilled if done properly. Floating slabs are not directly connected to the ground, and while caution is still needed, they do not have the same vulnerability as post tension slabs. Slabs on grade are typically supported by the ground and can also be drilled into with appropriate measures. Hence, the distinctive feature of post tension slabs and the implications of drilling into them highlight why this is the correct answer.