Option 4: (C), (B), (D), (A) -> Forms a complete, grammatically correct sentence: "Holidays at proper intervals are necessary for young people and also for those engaged in mental work as continuous work and no rest may cause serious health problems."
Hence, Option 4: (C), (B), (D), (A) -> The sentence follows a logical structure: (C) introduces the main subject and verb, (B) and (D) together specify who needs these holidays (young people and mental workers), and (A) provides the justification with the clause beginning with "as" -> correct