Answer:
D
Step-by-step explanation:
Sister chromatids will have the same form of alleles at the locus after S phase.
This is because during the S phae of the cell cycle, sister chromatids are replicated in preparation for cell division. The replication process is quite accurate such that all the alleles present on the initial chromatids are also replicated on the new sister chromatids. Hence, both new and old chromatids will have the same form of alleles at the locus after replication at the S phase. In other words, sister chromatids are exact copy of one another.
The correct option is D.