The second sentence is the correct one.
As but doesn't make sense, when would be saying at the same time and once would be saying afterwards.
check your question
the sentence that best describes the tone of the passage from the radio broadcast compared to the passage from the book is b. the radio broadcast has a more surprised tone.
consider the text evidence: in the passage from the book, the description is more matter-of-fact. it is simply describing the things resembling little grey snakes, but there is no urgency in the description. on the other hand, in the passage from the radio broadcast, there is definetely urgency (you can tell from the expression used, "good heavens", which clearly indicates surprise.)