California IDEX Practice Exam 2025 - Free IDEX Practice Questions and Study Guide

In a return-air plenum, certain items are restricted to ensure safety and efficient air handling. Wood blocking is not suitable in this environment primarily due to its potential to harbor pests and contribute to difficulties in fire safety. Since plenums often circulate air from various parts of a building, incorporating combustible materials like wood can pose a higher fire risk and may not meet code requirements.

On the other hand, fire dampers are specifically designed to close off airflow and prevent the spread of fire and smoke in duct systems, making them essential components for safety in such environments. Electrical cables are permitted in some instances, provided they are installed according to relevant codes and standards that address fire safety and insulation requirements. Water supply pipes might also be allowed in specific configurations that comply with building codes, depending on their placement and insulation. Therefore, wood blocking stands out as the correct answer for what is not allowed in a return-air plenum.