Patti, 

‘ La montagne ‘ is correct. 

In French, in the context of the ‘ endroit ‘,  ‘ la montagne ‘ is used in the general sense, whereas  the plural expression ‘ ( the ) mountains ‘ is used in English. Both the French and English titles for the exercise are appropriate to their native language use.

 ‘ Les montagnes ‘ would refer to specific mountains in this context, rather than being a general statement .

The dictation also uses the expression ‘ la montagne ‘ - “ mon endroit préféré “, ( comme elle a dit ).

See 2nd definition on both links, and also the definition and usage of ‘ à la montagne ‘ 

https://www.wordreference.com/fren/montagne

 https://www.larousse.fr/dictionnaires/francais/montagne/52476