The correct answer is - mountainous terrain.
The Balkan is a region that in general is dominated by mountains and high hills, so that is a factor that contributes that lot of types of farming are not suitable for big portion of the peninsula.
Still, the fact that the Balkan is dominated by mountains doesn't mean at all that it is not suitable for farming, since the valleys are pretty fertile, and despite the dry summers, there's a big variety of plants that prosper in here, like the vine trees, olives, tomatoes, peppers, to name some of the few.
Also, to see the mountains as places that are not not good for farming is a big misconception, as can easily be seen in the Balkan country of the Republic of Macedonia, which produces potatoes, hazelnuts, chestnuts, multiple types of berries, walnuts, in its mountainous parts.