The word which best describes the tone of the example is the following one:
C. Professional.
The speaker is interested in a position, and if he or she wants to be taken seriously, being sarcastic, casual or annoyed is definitely not the way to go. The speaker is professional to the point of outlining his previous experience in customer service roles. Note that no contractions are used either, as in "I am writing..." rather than "I'm writing". All of it belongs to the professional etiquette.