Which characters belong together in 区分开来？
区分 is the verb, to differentiate.

开 is a resultant complementary, apart.

来 is another resultant complementary, henceforth.

开来 commonly appear together in phrases similar to this one, but they do not form a word. (There is a word 开来 as in 继往开来 which is different).

The affinity of characters is (((区分)开)来) where only 区分 is a word.