Choose a. The stem means: "There are some health problems that will become more serious in the future if they are not treated in time." By analyzing the sentence structure, we can see that the adverbial clause is omitted here, and the subject problems and treat are passive, so item A is correct, and its complete form is when they are not handled in time. This sentence is actually a separate attributive clause. If not handled in time, larger attributive clauses will be separated in the future.