Her sketches are unusual because of their unusual perspective, radical use of color, and she uses unexpected subjects.

this sentence has an error in

a. verb tense.
b. parallelism.
c. punctuation.
d. modifiers.

The sentence stated above has an error in: B. parallelism. Parallelism is the use of identical grammatical form as to items of the same level of importance. 

Using parallelism, the sentence above should have been written this way:

Her sketches are unusual because of their unusual perspective, radical use of color, and unexpected subjects usage.

In this sentence, there is parallel structure in the use of adjective forms.

did you ever get an answer? i'm on the same question : (