What CSS property is used to arrange elements side-by-side?

The correct answer is related to the function of the CSS property that allows elements to be placed next to each other horizontally. The float property is specifically designed to position elements to the left or right of their container, allowing subsequent content to flow around them. When an element is floated, it is taken out of the normal document flow, which means that other block-level elements will behave as if the floated element is no longer there, enabling side-by-side arrangement.

The display property, while it can be used for layout purposes (especially with values like 'inline-block' or 'flex'), does not specifically target the behavior of elements flowing around one another in the same way float does.

Positioning using the position property affects how the element is placed relative to its normal position or to its nearest positioned ancestor. It doesn’t inherently facilitate side-by-side arrangement since it deals more with stacking order and specific coordinates rather than lateral flow.

The align property does not exist as a direct CSS property to control layout, and alignment often relies on other properties to have the desired effects on positioning elements relative to each other.

Therefore, float is essential for creating a layout where elements can be displayed side by side, effectively making it the best choice for the question.