Samuel Langhorne Clemens (Mark Twain) uses a fine humor style which is easily detected in extracts like:
<em>"Thish-yer Smiley had a mare; the boys called the fifteen minute nag(...) for all she was so slow and always had the asthma, or the distemper or the consumption, or something of that kind."</em>
<em>"...And he had a little small bull pup, that to look at him you´d think he warn´t worth a cent(...) his underjaw´d begin to stick out like the fo´castle of a steamboat..."</em>
<em>"...He ketched a frog one day, and took him home, and said he cal´klated to edercate him(...) and you bet you he did learn him, too.</em>
Twain is satirizing several aspects of American life, but specially the country "punks" who tend to speak at length about subjects that are close to them but are really unimportant an nonsensical.
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.
