Final answer:
Barry was constructing a horizontal line in his series of diagrams, which runs parallel to the x-axis and represents a constant y-value, being either positive or negative.
Step-by-step explanation:
In Barry's series of diagrams, he was constructing a horizontal line. A horizontal line in a coordinate system is represented by a constant y-value, indicating that it does not slope upwards or downwards. This type of line can either be at a positive value (above the x-axis) or at a negative value (below the x-axis), depending on its position in the coordinate plane.
Since the question specifically provides options that discuss horizontal lines with either positive or negative values, we can infer that Barry's line was not showing any increase or decrease (which would indicate a slope), but rather it was a straight line running parallel to the x-axis. Therefore, the correct answer is that Barry was trying to match sketches of a horizontal line.