A. Suddenly, Soon, Meanwhile, is the correct answer.
To me, it sounds most correct. Just imagine the sentence in your head, and look for if it sounds right, looks right, and means right:
"Suddenly, she heard a strange noise coming from inside."
"Soon, they were all huddled around Blush's stall."
"Meanwhile, the other cows stamped nervously at all the commotion."
The other transition words sound wrong when they fit with the sentences:
"Afterward, she heard a strange noise coming from inside."
"Long ago, they were all huddled around Blush's stall."
"Nevertheless, the other cows stamped nervously at all the commotion."
Hope this helps, and if you have any further questions about this problem, please ask me :)
