Respuesta :
The answer to your question would be that the best way to combine these sentences using correct punctuation is the following one: After I started working with a tutor, my grades improved. That is, the correct option would be C.
"After I started working with a tutor" is an introductory clause. Introductory clauses are dependent clauses that provide background information or "set the stage" for the main part of the sentence, the independent clause.