B. The use of words such as "unsustainable" and "unrecoverable" helps establish a formal style.
Al Gore's Nobel Prize acceptance speech is written in a formal style as it should be given the specific occasion of the speech. An informal speech would be given at a wedding, birthday party or some other more personal gathering. Nobel Prize acceptance speeches are often a way for the recipient of the Nobel Prize to continue to fight for the cause and work that earned him/her the Nobel Prize.
Option A is also wrong because the speech is not peaceful. It is a call to action. Option C is wrong because even though the speech should not be considered peaceful, it is also not angry. Option D is simply incorrect because a Nobel Prize acceptance speech should not be casual.
