Which CSS property is used to add space inside an element?

The CSS property used to add space inside an element is padding. When you apply padding to an element, it creates space between the content of the element and its border. This internal spacing helps in visually separating the content from the edges of the element, enhancing the overall readability and aesthetics.

Padding can be set uniformly or defined for each side of the element (top, right, bottom, left) individually. It plays a crucial role in designing a layout that feels balanced and provides a comfortable distance between the text or other content and the element's boundaries.

The other options serve different purposes: margin adjusts the space outside of an element, border creates a line around an element, and spacing is not a valid CSS property. Therefore, padding is the correct answer for creating internal space within an element.