The equation \( y = 2x + b \) is in the slope-intercept form of a linear equation, where \( m = 2 \) (the slope) and \( b \) (the y-intercept) can be any real number. The presence of \( b \) does not affect the linearity of the equation; it simply shifts the graph up or down.
Therefore, Joe's claim that the equation represents the graph of a straight line is true for all values of \( b \).
The correct answer is:
B. Joe's claim is true for all values of b.