The correct answer is D. A prepositional phrase means that it starts with a preposition. And in these examples, D is the only one that starts with a preposition "on".
I don't what it says can you right your question in the comments
I think personally and formally <em><u>True</u></em>.
This is because when asking for important information letters look more formal and appropriate.
Therefore the answer is True.
**hope this helps
<span>B) The text structure is problem and solution; the words safest and causing indicate this.</span>