The slope of the graph at x=0 is related to the value of b. It is also proportional to the value of <em>a</em>, which is the same for all but curve B. The red curve R has the largest slope at x=0, (much larger than 3/4 the slope of curve B), so curve R has the greatest value of <em>b</em>.
Similarly, the smallest value of <em>b</em> will correspond to the curve with the smallest (most negative) slope. That would be curve K. Curve K has the smallest value of <em>b</em>.
