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The 'nth-child' CSS selector is designed specifically to select elements based on their position among a group of sibling elements within a parent. This means that it can target elements that are in a specific order, allowing for more complex styling options based on their sequence.

The selector allows you to use a formula or specify a number to determine which elements to style. For example, `li:nth-child(2)` would select the second `<li>` element among its siblings, while `li:nth-child(3n)` would select every third `<li>` element starting from the first one. This ability to choose elements based on their order makes it incredibly useful for achieving specific designs without the need to add additional classes or IDs.

In contrast, other choices do not accurately describe the unique functionality of the 'nth-child' selector. Selecting only the first child, all children, or prefixing classes does not reflect the conditional positional capability inherent in 'nth-child'. Thus, the selection based on the order of siblings is what distinguishes this selector in CSS.