a. Rollerblading
Exercise that strengthens your heart and lungs is called aerobic exercise. And in order for aerobic exercise to work, you need to elevate your heart rate for at least 20 to 30 minutes. So let's take a look at the various exercises listed and see what each does.
Rollerblading - Moderate amount of effort, but happens over an extended period of time. Definitely aerobic, good candidate for the correct answer.
Pull-ups - This helps strengthen the upper back, shoulders, lats, and abs. Useful for building muscle strength, but not very aerobic. Not the correct answer.
Push-ups - Strengthen the chest, shoulders, and triceps. Another exercise for building strength, but not aerobic. Not the correct answer.
Rock Climbing - Helps build agility and strength. But also not very aerobic. Also not the correct answer.
So of the four exercises mentioned, only one of them involves sustained moderate effort over an extended period and that's rollerblading which is the correct answer.
