pH = - Log[\(H^+\)]. All we need is the concentration of the acid, and not the volume.

Option A : 5ml of \(\frac{M}{10}\)HCl; [\(H^+\)] = \(\frac{M}{10}\)= \(\frac{1}{10}\times M\) = 0.1M. pH = - Log(0.1)= 1

Option B : 10ml of \(\frac{M}{10}\)HCl;[\(H^+\)] = \(\frac{M}{10}\)= \(\frac{1}{10}\times M\) = 0.1M. pH = - Log(0.1)= 1

Option C : 15ml of \(\frac{M}{5}\)HCl; [\(H^+\)] = \(\frac{M}{5}\)= \(\frac{1}{5}\times M\) = 0.2M. pH = - Log(0.2)= 0.699

Option D : 20ml of \(\frac{M}{8}\)HCl; [\(H^+\)] = \(\frac{M}{8}\)= \(\frac{1}{8}\times M\) = 0.125M. pH = - Log(0.125)= 0.903

Among the options, option C tends to have the least pH value of 0.699