__________ is a system that rotates chemical stock to ensure they are used within their shelf life.

The correct choice is based on the principles of inventory management, specifically relating to perishable goods or materials with a limited shelf life. The concept of "First in, first out," or FIFO, ensures that the oldest stock is used first. This is particularly important in industries dealing with food, pharmaceuticals, or other materials that can degrade over time.

By implementing FIFO, businesses can minimize waste and ensure that products are consumed or utilized before they expire. This approach not only helps in maintaining quality but also complies with safety regulations, as using older stock reduces the risk of using expired materials.

In contrast, the other options represent different inventory management strategies that do not align with the goal of utilizing chemical stock within its shelf life. For instance, “Last in, first out” (LIFO) would mean that the most recently added stock is used first, which could lead to older items remaining unused until they expire. Random selection does not provide a systematic approach to managing shelf life efficiently, and "First in, last out" suggests an inverse method that again fails to prioritize timely usage of inventory. Thus, FIFO stands out as the most effective system for managing perishable stock.