​ Geometry help

B, RDF is a right triangle. 

Angle RDF is right, so triangle RDF must be a right triangle.

The proof is here: https://www.youtube.com/watch?v=z3YXznrwtp8 

I hope this helped,

Gavin

When many segments are named, and there is a lot of information conveyed in a succinct manner, I would suggest creating a diagram that represents the situation. The diagram below does just that:
 

What can you conclude about this diagram, and how can you apply it to the answer choices?