What is the required feature of an empty graduate’s base?

The required feature of an empty graduate's base that allows it to remain standing on an inclined surface of 15° from horizontal is essential for ensuring stability during experiments. This design feature is crucial because it helps prevent spills and accidents when measurements are being taken or when the graduate is placed on a surface that may not be perfectly level.

The ability to stand securely at an angle indicates that the base is engineered with a low center of gravity and appropriate weight distribution. This stability is particularly important in laboratory settings where precision is key, and any tipping could lead to inaccuracies in measurements or the loss of substances contained within the graduate.

In contrast, features like being made of plastic, detachable, or wider than the top do not directly address the critical aspect of stability on an incline, making them less relevant to the specific requirements for functionality in practical use.