What does the term "chromatid" refer to?

The term "chromatid" specifically refers to one of the two identical halves that make up a replicated chromosome. During the process of cell division, particularly mitosis and meiosis, chromosomes are duplicated so that each chromosome consists of two sister chromatids joined at a region called the centromere. These chromatids are identical copies of the chromosome that ensure that each daughter cell receives the same genetic material.

In contrast, the other options refer to different biological concepts. A molecule involved in protein synthesis relates to ribosomes and mRNA, while types of cell division refer to processes like mitosis and meiosis, which involve the separation of chromatids. The structure that helps in cell transport typically refers to cellular components like vesicles or transport proteins rather than chromatids. Hence, the definition of a chromatid is accurately reflected in the correct choice.