The equation \( y = 2x + b \) represents the equation of a straight line in slope-intercept form, where the slope is 2 and \( b \) is the y-intercept. This means that regardless of the value of \( b \) (whether it is zero, positive, or negative), the equation still describes a straight line.
Therefore, the best classification of Joe's claim is:
B. Joe's claim is true for all values of b.