The third one. A person uses another's lack of knowledge as proof his argument is correct.
This may be helpful:
https://literarydevices.net/fallacy/
Answer:
The Dursleys didn't have a direct conversation on this matter in the book. But when hagrid came to pick up Harry, and tells him about his parents then Harry mentions that the Dursleys have told him that his parents died in a car crash.
The reason I think they kept him in the dark is pretty obvious, that they hated the magic world, pretty much because of Petunia, as she was jeoulous that her sister (Harry's mother) was a witch and got admitted to Hogwarts and she didn't and they wanted to keep Harry away from that as they didn't want anymore of that "nonsense" and wanted to live a happy ordinary life.
Explanation:
<span>Squire
With him there was his son, a youthful squire,
A lover and a lusty bachelor,
With locks well curled, as if they'd laid in press.
Some twenty years of age he was, I guess.
In stature he was of an average length,
Wondrously active, aye, and great of strength.
He'd ridden sometime with the cavalry
In Flanders, in Artois, and Picardy,
And borne him well within that little space
In hope to win thereby his lady's grace.
Prinked out he was, as if he were a mead,
All full of fresh-cut flowers white and red.
Singing he was, or fluting, all the day;
He was as fresh as is the month of May.
Short was his gown, with sleeves both long and wide.
Well could be sit on horse, and fairly ride.
He could make songs and words thereto indite,
Joust, and dance too, as well as sketch and write.
So hot he loved that, while night told her tale,
He slept no more than does a nightingale.
Courteous he, and humble, willing and able,
And carved before his father at the table.</span>
