What is the primary characteristic of iteration in programming?

The primary characteristic of iteration in programming is that it executes instructions based on a specified condition. This means that iteration allows a set of instructions, or a loop, to be repeated multiple times as long as the condition remains true. This is especially useful for situations where the exact number of repetitions is not known beforehand, such as iterating through elements in a collection or performing an action until a certain criterion is met.

By using constructs like "for" loops, "while" loops, or "do-while" loops, programmers can efficiently process data, manipulate arrays, or execute a sequence of operations repeatedly without having to write the same code multiple times. The capability to run a block of code multiple times based on conditions enables developers to create dynamic and responsive applications.

In contrast, the other options do not accurately describe iteration: running indefinitely without stopping suggests an infinite loop, executing only once indicates a one-time function rather than iteration, and replacing all variables does not relate to the concept of repeating processes based on conditions.