Radiolabelled  DNA  was  allowed  to  replicate twice in a non radioactive environment. Which of the following is true?: (A) All the strands will have radioactivity, (B) Half of the DNA will have no radioactivity, (C) No strands will have radioactivity, (D) Three–fourth of the DNA replicated will have radioactivity
Answer is B. Semi conservative nature of DNA Replication proved by Meselson and Stahl states that half of the  parent strand is conserved during replication in the daughter strand.
After one replication all the DNA will have radioactivity.
After two replication half of the DNA will have radioactivity.

Ref : Harper’s illustrated biochemistry, 31st edition.