Explanation

(a)
The dispersal of the charge stabilises the carbocation. More the number of alkyl groups; the greater the dispersal of positive charge and therefore, more the stability of carbocation, thus $C_{2}H_{5}^{+} > CH_{3}^{+},\ O - CH_{3}$ is also an electron donating group, thus it will increase the stability of carbocation, hence, the correct order of stability is C>B>A.