The best way to tackle this question is to go through each answer and pointing out the errors.
"That book, I think is available at the library." A comma doesn't quite make sense here. A hyphen might be more appropriate.
"I'm exhausted after swimming, generally, speaking." This example would be correct if there wasn't another comma after "generally". This is because "generally speaking" is one phrase, and shouldn't be separated.
"There will be snow, I believe during tomorrow's festivities." This is the opposite of the previous answer. If there was a comma after "I believe", the sentence would be punctually correct. This is because "There will be snow during tomorrow's festivities" is the point of the sentence, but is interrupted with more information.
That leaves D. as the correct answer.
