The sentence refers to Sita having a pen and a pencil.
Option 1 is incorrect because the conjunction 'but' is used to introduce a phrase in the sentence that is contradictory to the previous phrase, but having a pen does not go against having a pencil, it is normal for a person to have both. Thus, option 1 is incorrect.
Option 3 is incorrect as having a pencil is not dependent on having a pen, one can have a pencil without having a pencil and vice versa. Thus, the conjunction 'so' cannot be used.
Option 4 is incorrect as the verb 'have' is in the plural form and the subject of the sentence is singular - 'Sita', thus option 4 is incorrect.