What device can be used to reduce friction when pulling cable into a vault?

The correct choice is cable lubricant. Using a cable lubricant is essential when pulling cable into a vault because it significantly reduces friction between the cable and the surface of the conduit or vault. This reduction in friction helps to prevent damage to the cable insulation and facilitates a smoother pulling process, making it easier and more efficient to install the cable.

Cable lubricants are designed to provide a slippery surface that allows the cable to move freely, allowing for longer cable runs and fewer instances of snagging or binding during installation. In many cases, using a lubricant can also help maintain the integrity of the cable over time, as it minimizes wear and tear that can occur during the pulling process.

While a flexible tube, pulling grip, and wire cable guide may assist in other capacities during cable installation, they do not specifically serve the purpose of reducing friction like lubricants do. A flexible tube might help guide the cable but does not provide lubrication. A pulling grip is used for securely fastening the cable to the pulling line but does not address friction, and a wire cable guide helps manage the cable's path but again lacks the friction-reducing properties of a lubricant. Therefore, the most effective and appropriate answer in this context is cable lubricant.