Did you read the official answer? It doesn’t match to the question, I think. If you have more time for changing to another good, your demand is more elastic, than in the short run. But I don’t think that you can conclude, that in the reverse, the more time you generally need to adjust, the more elastic your demand is. I did only get to c) by excluding a) and b), which seemed obviously wrong to me.
I did the same as Ew. a) and b) are def wrong as in its impossible to suggest that they are the right answer. My thinking with c) is that its in reference to the time since the last price change in that the adjustment has only actually occured meaning the time since the response to the last price change is recent rather then when the actual change occured.