This seems to be an almost duplicate of the previous question, https://forum.duolingo.com/comment/36654396?comment_id=38168401. Not teaching anything different and with exactly the same issue. So similar that I can just copy and paste what I wrote for that one.
I was offered two choices, with no English. They want you to say an rèidio 'the radio', although an also means 'their' so it could be 'their radio' (which is not a problem) but the alternative an rèidiothan 'their radios' is grammatically correct and makes sense, so this is not a valid question.
Note also that there is another valid translation,
Music is terrible on the radio.
I am sure there is regional variation, and I probably would not be consistent. I think there is probably a spectrum, with an sin the least likely to have the ann, then air an rèidio then a-muigh, then an-dè. Real life isn't hard and fast. My long answer was intended to show the logic, but people do not stick to logic all the time.
'Horrible' certainly comes within the range of meaning of grànnda in my view although I would say that 'ugly' was better. It is not listed by AFB, Dwelly or Mark. The only words they list for 'horrible' are uabhasach (which I agree is 'terrible'), a number of words I don't know, and oillteil, which I think is the best, but I am not sure how common it is.
I had translated the sentence as "There is horrible music on the radio" and it marked it as wrong and should be terrible. (I'm a musician (trumpet), and we use the words somewhat differently than the general public.) But in some cases uabhasach is used as 'horrible'. Just wondering what the subtle differences are, and whether I should use grannda instead of uabhasach... Not that it makes THAT much difference when speaking to other musicians...