Final answer:
The control structure used to reduce the number of lines of code for a repetitive action is called a loop.
Step-by-step explanation:
The control structure used to reduce the number of lines of code for an action that will need to be repeated many times is the repetitive control structure. This is also known as a loop. A loop allows a set of instructions to be repeated until a certain condition is met.
For example, in programming, a for loop can be used to repeat a set of instructions for a specific number of times. This saves the programmer from having to write the same code multiple times.
Another option is a while loop, which repeats a set of instructions as long as a certain condition is true. This can be useful when the number of repetitions is not known in advance.