All of the functions have a constant rate of change since they are linear functions with a slope (rate of change) that is equal to the coefficient of the independent variable. Therefore, all functions have a constant rate of change of their slope.
Option A: y = 6(2) = 12 has a slope of 6, which means it increases by 6 for every unit increase in x.
Option B: y = 2(3) = 6 has a slope of 2, which means it increases by 2 for every unit increase in x.
Option C: y = 3(4) = 12 has a slope of 3, which means it increases by 3 for every unit increase in x.
Option D: y = 4(2) = 8 has a slope of 4, which means it increases by 4 for every unit increase in x.
Therefore, option A has the most rapid rate of change with a slope of 6.