Ans. is 'c' i.e., Telomerase* A person can be young only until the person can replenish the losses periodically, for which the cells should be pushed into cell cycle regularly.* Telomeres are ends of chromosomes which help in maintaining the stability of our chromosomes. When we are born we are born with a particular telomeric length. As cells get into cell cycle, during replication, the telomeric end is progressively shortened. This happens because for DNA polymerase to replicate a DNA, we need to add RNA primers first. So ends of chromosome are replaced by RNA during replication. This RNA being labile gets lost.* After 50 to 70 cell divisions, the telomeric ends reach a critical or minimum length called as Hayflick limit. Beyond hayflick limit length of telomere, if the cells are allowed to get into cell cycle, the telomere length is not good enough to maintain the stability of chromosomes. The chromosomes accumulate mutation. To avoid accumulation of mutation, once a cell has reached the critical length of telomere, the cells are not allowed to get into cell cycle any more. When the cells are not able to get into cell cycle, the losses are not being replenished and that is when ageing begins.* To avoid this we are provided with a telomerase enzyme. Telomerase is a reverse transcriptase. IT has RNA dependent DNA polymerase activity. Based on the RNA fragments left back in the terminals as a result of replication, Telomerase synthesises and replaces DNA fragments. Hence the length of telomeric ends can be maintained and ageing is prevented.* Hence telomerase is an antiageing enzyme.