Final answer:
The correct answer is 1) Chromatids, which are formed through DNA replication and are attached at a centromere, becoming sister chromatids.
Step-by-step explanation:
Genetically identical chromosomal units that are formed through DNA replication and are attached at a centromere are called sister chromatids. During the DNA synthesis phase, or S phase, of interphase, the DNA in each chromosome is replicated to form two identical copies. These copies, now termed sister chromatids, stay connected at a central region known as the centromere, which appears as a constricted area on the chromosome. Cohesin proteins help hold these sister chromatids together, ensuring they remain attached until they are separated during mitosis or meiosis. Thus, the correct answer to the student's question is option 1) Chromatids.