Question

Compare the stability of carbocation and carbanion

Answer 1

Stability order of carbocations increases as we move from primary to tertiary cation due to +I effect of methyl groups there is a redistribution of positive charge all over the molecule which reduces the intensity of positive charge on central carbon and increases the stability of the molecule.

Stability order of carbanions decreases as we move from primary to tertiary anion because due to +I effect of methyl groups there is an increased intensity of negative charge on central carbon of tertiary carbanion which further makes it unstable.

Answer 2

Carbocation stability
Carbocations prefer a greater degree of alkyl substitution. Even more so, carbocations prefer to be in the allylic position.
Carbanion stability
Carbanions prefer a lesser degree of alkyl substitution. Even more so, carbanions prefer to be in the allylic position.