One never changes spoken contractions (e.g. y'all, ain't, don't, can't, it's, etc.), which means that you should transcribe them as they are spoken.
However, you should change slang words. When someone says "gotcha" , "wanna", or "gonna", they must be transcribed as "got you", "want to" and "gonna" respectively.
Original Speaker: "I totally wanna go down to the library and get a book, but I can't because it's closed now."
Clean Verbatim File: "I totally want to go down to the library and get a book, but I can't because it's closed now."
Note that the spoken contractions "can't" and "it's" are transcribed as they were spoken. However, the slang "wanna" has been transcribed as "want to".
Answer: (b) Spoken Contraction