Answer: d. 29%
Explanation:
The mixture of the two concentrations should equal a concentration that is between the concentration percentages of the two solutions.
This is because the lower concentration percentage will pull the higher one down while the higher concentration will pull the lower one up. It will not be possible to get a mixture concentration that is lower than the lower concentrated acid or higher than the higher concentration acid.
Mixture concentration must be between 15% and 25%.
Option D at 29% is therefore not possible.