Here is the argument, but with the correct portion in bold (Kamal2014 seem to have left out the bold in original post)

One of the limiting factors in human physical performance is the amount of oxygen that is absorbed by the muscles from the bloodstream. Accordingly, entrepreneurs have begun selling at gymnasiums and health clubs bottles of drinking water, labeled "SuperOXY," that has extra oxygen dissolved in the water. Such water would be useless in improving physical performance, however, since the only way to get oxygen into the bloodstream so that it can be absorbed bye the muscles is through the lungs.

Which of the following, if true, would serve the same function in the argument as the statement in boldface?

A) the water lost in exercising can be replaced with ordinary tap water

B) the amount of oxygen in the blood of people who are exercising is already more than the muscle can absorb

C) world-class athletes turn in record performance without such water

D) frequent physical exercise increases the body's ability to take in and use oxygen

E) lack of oxygen is not the only factor limiting human physical performance

The answer B
But I feel the answer is D.
Please help. Please quote the reason

This is a special kind of bold-faced question with just one statement in bold and the answer does not describe the role but rather the correct answer replaces the portion in bold.

So why, is B the correct answer to this question?

It is because we are focused on the FUNCTION played by the boldfaced portion. The function is to explain why super-oxygenated water is useless to the body and the bold-faced portion does this.If the "only way to get oxygen into the bloodstream so that it can be absorbed by the muscles is through the lungs" then it will not be helpful at all to have extra oxygen in the water!

Answer choice B also gives a reason why the super-oxygenated water is not needed - too much oxygen already! "the amount of oxygen in the blood of people who are exercising is already more than the muscles can absorb" Focus on the oxygen...that is the key. Why the oxygen in the water is not useful.

Choice D says "frequent physical exercise increases the body's ability to take in and use oxygen." This does not serve the same function. Remember the phrase just before the bold says "Such water would be useless in improving physical performance" this means we want the superoxy water to be completely useless. Choice D does not do this. If the body gets better at taking in oxygen, as stated in D, maybe it takes in oxygen from this water, right?