Raceways or cable trays containing electric conductors must not contain any pipe for other services besides ______.

The correct answer is focused on the fact that raceways or cable trays that are specifically designated for electric conductors are permitted to contain only electrical conductors. This requirement is grounded in safety and code compliance, as mixing different types of conductors or services within the same raceway can lead to potential hazards, complications in maintenance, and interference between systems.

By allowing only electrical conductors in raceways or cable trays designated for them, it facilitates a clearer understanding of the system's design and maintenance protocols. It also ensures that the conductors are managed and protected appropriately according to the National Electrical Code (NEC) and other relevant standards, minimizing the risk of accidental damage or operational issues.

The other options suggest scenarios where different systems could potentially coexist within the same raceway structure, which does not conform to standard practices aimed at preserving safety and functionality in electrical installations. Options suggesting authority approval or design specifications introduce possibilities that could compromise the integrity of the installation if not carefully managed and do not align with the core principle of maintaining electrical systems separately from other utility services.