The interval between values is not interpretable in an ordinal measure. In interval measurement the distance between attributes does have meaning. For example, when we measure temperature (in Fahrenheit), the distance from 30-40 is same as distance from 70-80. The interval between values is interpretable.
Stretching increases flexibility, sit-ups are for abdominal endurance, and distance running are for leg muscle endurance, so the correct answer is pull ups.