To mark various angles accurately, which tool would a mason use?

A bevel square is specifically designed for marking and measuring angles accurately, making it the ideal tool for a mason when precision is crucial. This tool features an adjustable arm that allows the mason to set it to various angles, often used in cutting materials or ensuring that components fit precisely together at specific angles.

While other tools mentioned serve important functions in masonry, they do not have the specific capabilities to accurately mark angles. The level, for example, is used to ensure that surfaces are perfectly horizontal or vertical, but it does not help in measuring or marking angles. A trowel is a tool primarily used for applying and shaping mortar, lacking any angle measurement features. A chalk line is beneficial for marking long, straight lines and is more useful in aligning components but is not designed for measuring angles. Thus, the bevel square stands out as the best choice for the task of marking various angles accurately in masonry work.