Final answer:
Cohesin is the protein that holds sister chromatids together at the centromere after DNA replication until they are separated during anaphase in mitosis.
Step-by-step explanation:
The protein responsible for holding sister chromatids together after replication is cohesin. Cohesin acts as a physical bridge that maintains the connection between the sister chromatids through G2 phase and into mitosis, specifically up until they are separated during anaphase. This connection is most pronounced at the centromere, which is a highly condensed and constricted area of the chromosome that is visible under a light microscope when the chromosomes are compacted. The correct answer to the student's question is b. cohesin.