Explanation:

Let's look at each sentence to form the right order:

- B: Talks about the usefulness of honest advertisements. It's a good opening.

- A: Follows naturally by explaining how honest ads can help in decision-making.

- D: Introduces the downside, mentioning harmful ads that deceive.

- C: Specifically points out that teenagers are most vulnerable to deceptive ads, which flows logically after "D."

Now, let's see the options:

- Option 1, CDAB: Starts with the vulnerability context, which doesn't set the scene well.

- Option 2, BADC: Begins with positive aspects, then the negative, ending with vulnerabilities, fittingly.

- Option 3, BCAD: Ends abruptly on how ads help you, missing the critical note.

- Option 4, CADB: Starts with vulnerabilities, ending positively, an awkward tone switch.

So, the correct order is Option 2, BADC.