Which of the following carbocation is most stable?

(a)

(b)

(c)

(d)

Correct option is (b)

is a tertiary carbocation(the carbon with a positive charge is attached with 3 methyl groups). A tertiary carbocation is the most stable carbocation due to the electron releasing effect of three methyl groups (+I effect). An increased +I effect by three methyl groups stabilizes the positive charge of the carbocation.

Order of stability of carbocation:- primary<secondary<tertiary.