Nonlinear would be any function where both values are not changing by some fixed amount. In B, f(x) increases by 1 for every time x increases by 1 so that's linear. For C, f(x) increases by 2 for every time x increases by 1, so that's linear. For D, f(x) increases by 3 for every time x increases by 1 so that's also linear. For A, f(x) does not always change by the same amount, so it's nonlinear.
