Final answer:
The word "upright" is associated with the position that is vertical or perpendicular to the ground. It matches option a) vertical. This term represents a straight alignment like that of a standing person or a tall building.
Step-by-step explanation:
The word "upright" matches with the option a) vertical. When something is upright, it is in a position that is perpendicular to the ground. This is similar to being vertically upward in the coordinate system, as opposed to horizontal, slanted, or circular positions. An upright posture means standing straight with good alignment, like a skyscraper that rises vertically, or a tree that grows straight up from the soil.