Final answer:
Carbocation stability increases from methyl (least stable) to primary, secondary, and tertiary (most stable) due to the inductive effect of alkyl groups.
Step-by-step explanation:
The stability of carbocations increases with the number of alkyl groups attached to the positively charged carbon. The order of stability is as follows: methyl (least stable), primary (RCH₂⁺), secondary (R₂CH⁺), and tertiary (R₃C⁺) (most stable).
This order is based on the ability of alkyl groups to donate electron density through the inductive effect, which helps to stabilize the positive charge on the carbon atom. Thus, a tertiary carbocation, which has three alkyl groups stabilizing it, is more stable than a primary or methyl carbocation, which has fewer or no such stabilizing groups. Therefore, the stability of carbocations increases in the following order: methyl < primary < secondary < tertiary.