Final answer:
To resize an object from the center and maintain its proportions in software like Photoshop, hold down the Shift and Alt keys (or Shift and Option keys on Mac) while dragging a corner handle.
Step-by-step explanation:
To resize an object from the center and constrain proportions in graphic design software such as Adobe Photoshop, you generally hold down the Shift key while also holding down the Alt key (Option key on Mac) as you drag a corner handle of the object's bounding box. Holding the Shift key ensures that the object's proportions remain consistent, meaning the width and height are scaled equally. Holding the Alt (Option on Mac) key resizes the object from its center outward, rather than from a corner.