Changing direct speech into reported speech requires three adjustments at once. The reporting verb takes the listener as a direct object when “told” is used, the pronouns shift from the first person to the third person, and the tense of the quoted words moves one step back because the reporting verb is in the past.

Applying all three to the quoted words gives the following result. The reporting verb takes the listener directly without a preposition, the first person pronouns become third person pronouns with the matching possessive, and the present continuous and the present tense of the quoted sentence both move into the corresponding past forms.

One version keeps the first person pronouns and the present tense untouched, so no backshift has taken place at all. Two versions insert a preposition after the reporting verb, which that verb does not take before the listener, and they also replace the continuous form with a perfect continuous that the speaker never used.

Gaurav told Saurabh that he was not taking the examination because he was not very confident about his studies option is correct.