The type of tone a writer should use is respectful.
Explanation:
A job cover letter is the first view a company has of you as an applicant, then you have to make it as a representation of the summary of your best qualities, since this will be determining if they would like to interview you or not, the other options show a kind of personality that is normally avoided by employers at the moment of hiring.
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.
