In standard position, which axis does the terminal side of a $90°$ angle lie on?

• A. The positive $x$-axis
• B. The negative $y$-axis
• C. The negative $x$-axis
• D. The positive $y$-axis

Answer: (D)

Verified step by step guidance

1. Recall that an angle in standard position is measured starting from the positive x-axis and rotating counterclockwise.
2. Understand that a 90° angle corresponds to a quarter turn from the positive x-axis.
3. Visualize or sketch the coordinate plane and rotate the initial side (positive x-axis) by 90° counterclockwise.
4. Identify the axis where the terminal side lies after this rotation; it will be along the positive y-axis.
5. Conclude that the terminal side of a 90° angle in standard position lies on the positive y-axis.