The correct statement is: Barring mutation, the two sister chromatids must be identical.
Sister chromatids are the two identical copies of a chromosome that are formed by DNA replication. They are joined at the centromere prior to cell division (including meiosis), and they are genetically identical to each other, provided that no mutations have occurred during DNA replication.
The statement "These chromatids make up a diploid chromosome" is incorrect because sister chromatids represent two copies of a single chromosome, and together they do not constitute a "diploid chromosome" but rather are part of the diploid state of the organism. A diploid organism has two sets of chromosomes, one from each parent, and each chromosome consists of two sister chromatids after DNA replication.
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