The real-world problem most likely to involve an irrational number is:
a. finding the side length of a square based on the area.
When you find the side length of a square given its area, you take the square root of the area. For example, if the area of the square is 2 square units, the side length would be \( \sqrt{2} \), which is an irrational number.
The other options may involve rational numbers or straightforward arithmetic without yielding an inherent irrational outcome.