Notice f(x) is nonnegative for an interval covering the left part of the graph, and negative on the right part of the graph. That means its antiderivative is increasing for the left interval, then decreasing for the right interval. This applies to A(x) and C(x), but not B(x).
There are only three points where A(x) has a horizontal tangent line: at its global maximum and the endpoints of the interval shown. By contrast, C(x) has a horizontal tangent line in four places: at its global maximum, at its inflection point, and at the endpoints of the interval shown. Since f(x)=0 four times (and these line up with the horizontal portions of C(x)) we conclude C(x) is the antiderivative of f(x).