The s phase of the cell cycle results in the production of two identical copies of each chromosome, known as

In a cell cycle, the S phase or the synthesis phase is the part in which the DNA (deoxyribonucleic acid) is replicated. This result in doubling the amount of DNA present in a cell.  

This replicated DNA is later condensed to form the chromosome. These chromosomes have two identical copies of DNA bind together with help of a centromere. The identical copies in each chromosome is known as sister chromatids, which get separated at the time of Anaphase.  

Hence, the given blank can be filled with Sister chromatids.  

a and c are the answeres

when it is at an intermediate population density