Which constraint causes lines drawn to be parallel to the y-axis?

The vertical constraint is used in Autodesk Inventor to ensure that lines are aligned parallel to the y-axis. When this constraint is applied to a line, it locks the line's orientation, making it impossible for the line to tilt or rotate out of that specified vertical alignment. This is particularly useful in maintaining consistent dimensions and layout designs in a sketch, as it prevents any accidental adjustments that might shift the line away from the vertical position.

In contrast, the horizontal constraint would ensure lines are parallel to the x-axis, while the diagonal constraint allows for lines to be at any angle rather than fixed to vertical or horizontal positions. The perpendicular constraint is used to maintain a right angle between two lines or edges, which does not specifically dictate vertical or horizontal alignment. Thus, the vertical constraint stands out as the correct choice for ensuring lines are parallel to the y-axis.