When installed in the same cable tray, cables operating at over 600 volts shall be of which type?

The correct choice indicates that when dealing with cables operating at over 600 volts installed in the same cable tray, they must be of the MC (Metallic Cable) type. This requirement is primarily due to the construction and safety characteristics of MC cables.

MC cables are designed to handle higher voltage levels safely and are equipped with a metal sheath that provides both protection against physical damage and a path for grounding. This is crucial for minimizing the risk of electrical hazards such as short circuits or fires that can arise from interference or damage to the insulation.

In contrast, other cable types listed have limitations regarding their voltage ratings or physical protection. AC (Armored Cable) is typically more flexible and suited for specific applications but may not offer the same level of protection at higher voltages in a tray scenario. BX (an older term often referring to flexible metallic conduit) and NM (Non-Metallic Sheathed Cable) are generally not suitable for installation in environments where they could be exposed to damaging conditions or where higher voltage ratings are required, particularly beyond 600 volts.

Thus, selecting MC cable in this context ensures compliance with safety codes and standards for electrical installations, especially when multiple cables are housed together in a cable tray.