Translation:I think that the most important thing is that she feels safe.
This may be another example of how English has "evolved" over the years: we were taught to maintain the sense of the subjunctive in English by translating the above "....that she feel safe." "Feel" being in the subjunctive mood. Now it seems perfectly acceptable to use the present tense.
But "I think that the more important thing is that she feels safe." = "Penso che la cosa più importante sia che lei si senta al sicuro." is marked wrong. Surely "I think that the most important thing is that she feels safe." would be translated as "Penso che la cosa la più importante sia che lei si senta al sicuro."
Paul, I agree with Skender. If it were the comparative it would be "una cosa piu importante". In the exercise: the definite article before the cosa puts it uniquely ahead of the other(s) making it superlative, at least so in Italian. The second definite article is apparently redundant to the Italians.
Even in your comparative English sentence that definite article makes it uniquely more important = the most important (thing) between or among what you are comparing. To me "The more important thing..." seems just as acceptable a translation as "the most important thing" however I am not Italian and will defer to native speakers.
Here is one of many sites that address the Italian relative superlatives. It is about 2/3 of the way down: http://icebergproject.co/italian/2014/06/relative-and-absolute-superlatives-in-italian-or-the-best-the-worst-and-happiest/
I'm noticing that the verbs "sia" and "senta" in the subordinate clause are both in the subjunctive. I don't see this happen very often. Is it normal in Italian (also Spanish?) that, when a sentence calls for a subordinate clause with verbs in the subjunctive, that ALL verbs in that clause be in the subjunctive? Or is it just the verbs which need to be highlighted because of their "separation from reality" (their "subjunctiveness," so to speak)?
I think you would use a definite article to turn the adjective "importante" into a noun, which would translate to "what is important" or "the important thing": "Penso che il più importante sia che lei si senta al sicura." Using "la cosa" here would just be a more literal way of making the statement.
You could start with this: https://forum.duolingo.com/comment/8783716/Italian-Subjunctive-Guide
Если бы вы действительно предоставили точное содержание основного перевода на английский, он был бы принят. Я предполагаю, что вы допустили какую-то ошибку, например, изменили порядок слов или пропустили статью. Обе вещи очень важны на английском языке. Пожалуйста, напишите на английском в следующий раз.
(If you had really provided the exact content of the main translation to English, it would have been accepted. I assume you have made some mistake, such as changing the word order or omitting an article. Both things are very important in English. Please write in English next time. )