The audio is in fact saying "forté" or forte how we would pronounce this word in English in both the regular and slowed speeds.
the audio pronounces "fortée" instead of "forte"! Please, repport it. January 2018
I think "strong" is better than "high" here not only because it's a more direct translation, but because a "strong demand" makes perfect sense in English too.
On this course, Duo teaches you French. English is the working language just used to help you memorize how things are told in French.
All the hints are verbs. Though it can be reasoned from context that "demande" is a noun that means "demand", I do not think this is ideal.
Why would "The demand" be incorrect...
The choice of preposition ('at' or 'in') depends on the specific English sentence you are using as a translation. In particular, if you're sentence was ". . . this season," then 'in' or 'during' would be natural, but 'at' would not.
The inclusion of the preposition "in" is clumsy English. We would say "Demand is high this season"