Who's going to explain the complex concepts in this chapter, it's really confusing for me.
Correct: B
The first part of the sentence, 'Who's going to explain the complex concepts in this chapter,' is a complete interrogative sentence. The second part, 'it's really confusing for me,' is a separate independent clause. These two distinct sentences cannot be joined by a comma (which would create a comma splice). A question mark should end the first sentence, and the second sentence should begin with a capital letter. Option B correctly uses a question mark and starts a new sentence.