A 3D object must be mentally rotated 90 degrees around its vertical axis. Which side of the object was facing right becomes which side after this rotation?

MENTAL ROTATION DIRECTION is critical and varies based on the specified rotation direction. CLOCKWISE 90 DEGREES ROTATION (viewed from above): What was facing RIGHT → now faces TOWARD YOU (front); what was facing TOWARD YOU → now faces LEFT; what was facing LEFT → now faces AWAY (back); what was facing AWAY → now faces RIGHT. COUNTERCLOCKWISE 90 DEGREES ROTATION (viewed from above): What was facing RIGHT → now faces AWAY (back); what was facing AWAY → now faces LEFT; what was facing LEFT → now faces TOWARD YOU; what was facing TOWARD YOU → now faces RIGHT. VERTICAL AXIS ROTATION affects which face you see from the front, back, left, right — but TOP and BOTTOM stay the same. This is why Option C is most accurate — the result depends entirely on the direction of rotation.

Building the mental model: Physically hold any rectangular object (book, phone, box); rotate it and observe which face moves where; then try to predict each position before doing it physically; eventually you can do this purely mentally. This physical-to-mental training is the fastest way to improve on mental rotation tasks.