Which of the following is the strongest base?

Arrange the following in the order of increasing stability:

${\mathrm{PhC}}^{+}{\mathrm{H}}_2, {\mathrm{Ph}}_3{\mathrm{C}}^{+}, {\mathrm{Me}}^{+}, {\mathrm{Ph}}_2{\mathrm{C}}^{+}\mathrm{H}$

Out of the four ${\mathrm{pK}}_{\mathrm{a}}$ values $3.75, 9.89, 15.54$ and $19.30,$ which has the highest ${\mathrm{pK}}_{\mathrm{a}}$ value?

The most acidic of the following substances is: