The stability of carbocations ($R^{+}$) follows the order: $3^{\circ }>2^{\circ }>1^{\circ }>\text{Methyl}$.

• Reasons:

1. Inductive Effect (+I): Alkyl groups are electron-donating. More alkyl groups around the positive carbon disperse the partial positive charge better.
2. Hyperconjugation: The number of $\alpha$-hydrogens increases in the order $1^{\circ }$ ($3$), $2^{\circ }$ ($6$), $3^{\circ }$ ($9$). More hyperconjugative structures lead to greater stability.