Translation:That class is not easy.
Sometimes in Japanese, people will use その to express exactly which class they are referring to. Effectively emulating our direct article.
Duolingo has pretty consistently translated この as "this" , その as "the" , and あの as" that"
その should definitely be "that" and not "the", though if you were really loose about translation of course it would be okay to translate any of these words as "the" some of the time. It refers to something close to the listener generally in the same way "that" does in English.
I believe その is used as "the" when speaking about something already referenced in the conversation.
Exactly. So if either "the" or "that" is not accepted, please report.
優しい can mean easy, but it also means kind. 簡単 is a better word for easy if you ask me.
Tooltip says じゅぎょう can translate as "class" or "lesson". Why is "The lesson is not easy" incorrect?
Yes Duolingo, lesson is a correct answer
授業 1. lesson; class work; teaching; instruction
その授業は簡単ではありません。そのじゅぎょうはかんたんではありません。 I was sat with a native speaker during this question and they told me that this expression would not be used by them. They would use the word 簡単(かんたん）for easy instead. In fact both of them laughed at this expression for its poor Japanese. I put it in polite form but usually this sort of conversation would be with peers.