You use preterite to describe an action that took place and was fully done, Ex. José entro a la habitación, you can see John went into the room, he completed the action. On the other hand, imperfect tense doesn't specificate if the action was completed, Ex. José entraba a la habitación, it doesn't tell us if José fully entered the room.
Your formal singular in Spanish is -tu singular formal-
The correct is b) <span>No he hecho lo por mucho tiempo</span>
Se , lo .. would be the pronouns
