Q.Arrange methyl, primary ($1^\circ$), secondary ($2^\circ$) and tertiary ($3^\circ$) carbocations in order of increasing stability, and give the reason for the trend.

A carbocation's empty $p$ orbital is stabilised by electron density donated from adjacent $\text{C--H}$ (and $\text{C--C}$) sigma bonds via hyperconjugation, and by the mildly electron-donating inductive effect of alkyl groups relative to hydrogen. Both effects grow with the number of alkyl groups directly attached to the cationic carbon: a tertiary cation has three such alkyl groups feeding it stabilisation by both mechanisms, a secondary cation has two, a primary cation has one, and the methyl cation, with no adjacent carbon at all, …

Count the number of alkyl groups directly bonded to the cationic carbon -- each one adds both a hyperconjuga …

Forgetting that benzylic/allylic cations, stabilised by resonance, are generally even more stable than a simple tertiary alkyl cation -- alkyl substitu …