Dimitra - this sentence discussion looks like a Greek-to-English translation exercise or a Greek "type what you hear" exercise - I don't think we can add alternatives in listening exercises.
If the voice says μία (with two syllables), the learner has to type μία. If the voice says μια (with one syllable), the learner has to type μια.
There's no substantive difference between μία and μια, just a pronunciation difference which is totally preferential, and situational. Greeks switch back and forth between the two pronunciations all the time.
It's comparable to English's pronunciations of "the" and "a." Sometimes people pronounce "the" like "thee" and other times like "thuh"; sometimes "a" is pronounced like "uh" and sometimes like "ay".
"Αυτή είναι μία άσπρη γάτα." can be:
It is a white cat. This is a white cat. She is a white cat.
These are all correct and accepted translations. I don't know why RogerDear's sentence was not accepted, there must have been another mistake. I'm really sorry we didn't see it sooner to explain.