Translation:I like the mountains.
Correct answer: "I like the mountain." (singular)
Mountains (plural) would be "الجبال".
You are technically correct, but in the Arabic expression, it could also be implied that you like to live in the mountains, in general, as opposed to the city. In such idiomatic usage, Arabic talks about the mountains as a singular entity while English uses the plural form.
Having said that, I agree with you in that, for the sake of making learning Arabic easier, the literal translation using the singular English form should be used as the default answer, and the other should simply be accepted. That would make teaching plurals easier and less confusing.
Here it's more like: I like the city, as in I like the city environment in general.