I would say the correct answer is girls had laughed a little nervously.
Based on this clause, we can deduce something about their personality, and that is that they are happy kids who want to laugh all the time but they are not allowed to. The other options are either direct characterization or not characterization at all.
Hannah's wedding was a day of great hilarity because she got married to the person of her dreams.
The abbreviation after a word in the dictionary tells you what kind/ part of speech the defined word is.
Hope this helps! :)
