Final answer:
The definition given corresponds to a system of linear equations, which refers to answer option 'a) Linear equations'.
Step-by-step explanation:
The definition 'a set of two equations of the form ax + by = C, where x and y are variables and a and b are not both zero' refers to a system of linear equations. These equations represent straight lines on a graph, and when they are paired, the intersection of these lines (if one exists) is the solution to the system. Each equation in a linear system can often be rewritten into slope-intercept form (y = mx + b), where m is the slope and b is the y-intercept. By comparing the provided definition with the characteristics we know of linear equations, it is clear that the correct answer is 'a) Linear equations'.