Entire sentences are not masculine or feminine. Do you mean why is big feminine? If that is what you mean it is because problem is feminine (if you ask my husband he will agree that problem is always feminine).
My answer: التلفاز معطل و ه‘ذه مشكلة كبيرة
Correct solution: اَلْتِّلْفاز مُعَطَّل وَهٰذِهِ مُشْكِلة كَبيرة.
My answer was rejected.
The version of ه‘ذه I wrote in my answer has always previously been accepted. My leaving a space after و has also always previously been accepted.
Experiencing this again on a different lesson with the same quiz question. I think there is a problem again here with the full-stop / period at the end of the sentence. I do not know what else it might be.