The answer is: none of the above.
The options given - swimming, running and bike riding - are examples of aerobic exercises, commonly referred to as cardio exercises nowadays.
Flexibility exercises demand precisely that: flexibility - the ability to bend, to move one's joints through a range of motion. Exercises that help increase one's flexibility are stretching and yoga, for example. However, stretching is a common and necessary practice among athletes since it helps prevent injuries.
