To quickly resize an object while maintaining its proportions, which key should you hold down?

When resizing an object in Adobe Illustrator while wanting to maintain its proportions, holding down the Shift key is the correct method. This ensures that when you click and drag a corner anchor point, the width and height of the object are scaled evenly, preventing any distortion of its original shape. This function is key for artists and designers who need their illustrations to remain visually accurate, especially when working with logos and other graphic elements that rely on specific ratios.

In this scenario, the other keys have different uses: the Alt key can be employed to resize an object from its center outward, which might not preserve the proportions unless combined with the Shift key. The Ctrl key is primarily used for precise adjustments and transformations in other contexts, such as constraining movement to specific angles or dimensions. The Spacebar is typically used for temporarily switching to the Hand tool to move the document view while working on a project without changing the selected items. Thus, the Shift key is essential for proportionate resizing, making it the best choice for this action.