Electrophilic addition reactions proceed in two steps. The first step involves the addition of an electrophile. The major intermediate formed in the first step is -

H3C-HC=CH2+H+?

1. 2° carbanion

2. 1° carbocation

3. 2° carbocation

4. 1° carbanion

Hint: Stability of carbocation

When electrophile attacks CH3-CH=CH2 delocalisation of electrons can take place, in two possible ways
Carbocation X is 2o and carbocation Y is 1o. As 2° carbocation is more stable than 1° carbocation. Thus, option third correct.