Arrange the carbocations in decreasing order of stability: (P) $\mathrm{CH}{}_3\mathrm{CH}{}_2{}^{+}$, (Q) $(\mathrm{CH}{}_3){}_3\mathrm{C}{}^{+}$, (R) $\mathrm{CF}{}_3\mathrm{CH}{}_2{}^{+}$, (S) $(\mathrm{CH}{}_3){}_2\mathrm{CH}{}^{+}$.

The +I effect of alkyl groups stabilises a carbocation, so tertiary > secondary > primary. The strong -I effect of the $\mathrm{CF}{}_3$ group destabilises $\mathrm{CF}{}_3\mathrm{CH}{}_2{}^{+}$ the most, giving Q > S > P > R.