Which structure is commonly used to implement iteration in programming?

A looping structure is specifically designed to facilitate iteration in programming. It allows a block of code to be executed repeatedly based on a specified condition. Common types of looping structures include 'for' loops and 'while' loops, which enable programmers to efficiently perform repetitive tasks without having to write the code multiple times. This is crucial in situations where a task needs to be repeated a number of times or until a certain condition is met.

The other options serve different purposes. An if statement is used for making decisions based on conditions but does not inherently repeat code. A conditional expression evaluates to a value based on a condition, but it also lacks the ability to execute statements multiple times. A return statement is utilized to exit a function and send a value back to the calling context, but it does not facilitate iteration. Therefore, a looping structure stands out as the correct answer for implementing iteration.