Explanation Correct Answer is B. 0.0001
Explanation
The molarity of a solution is a measure of the concentration of a solute in a solution. In this case, the pH of the solution is given as 4, which indicates that the concentration of H+ ions in the solution is 10^-4 M. Since hydrochloric acid (HCl) is a strong acid that dissociates completely in water, the concentration of HCl in the solution is also 10^-4 M. Therefore, the molarity of the solution is 0.0001 M.