What unique instrumentation change did beethoven’s symphony no. 9 use that, according to the website, was a “radical definition of the form” of a symphony?

I would say that the answer is the introduction of a chorus into the form of symphony, which had previously been considered a purely instrumental genre. Beethoven was the first composer who dared to do this and was followed by Mahler, Bruckner, Shostakovich and other notable composers.

Opposition of two melodic ideas is the  right answer 

Riffin tells a story about photographing elephants in a wildlife sanctuary in africa. he says that the photographer brought back more than just an amazing story.