You could still call a quill pen a "pen" in english, too. "Quill pen" is to emphasise that it's a pen with a literal quill (say, to distinguish it from nearby pens). Writing quill pen is therefore too specific - "pluma" could be referring to various kinds of fountain pens that may or may not have an actual quill, and sometimes may refer to general pens depending where you are.
Little advice from real life in Spain - do not use the word "pluma" for pen. People will usualy just laugh at you. In Spain it is a slang expression to say "camp" realted to gay people. If you say "tienes una pluma", peaople can get easily offended because to them you say "you are really camp (gay camp)". They use it more to make fun or jokes. They never use this word for "pen" :-)
Quill is one correct translation. They are trying to teach you a more broad word for 'pen' though. Historically, (plume) is exactly the etymology of this word's usage for 'pen'.This did refer to quill pens, yes. I'm sure you know that pluma is Latin for plume/feather. Today, it is more broad than 'quill' or 'plume'. Any pen with a nib is a 'pluma' to all Spanish speakers and some even use the word for all pens.
just a few questions ago the sentence was, "tienes reloj?" no article after tienes. and that's the way it's supposed to be, see http://spanish.about.com/od/adjectives/a/indefinite.htm
maybe i'm missing something. that' happens often enough.