A chromosome is a thread-like structure that is composed of DNA and protein. It carries all the genetic materials of an organism. This is passed from parent to offspring.
Chromatid is one of the halves of a chromosome.
A chromosome contains only one chromatid. When the cell cycle occurs then after the completion of the S-phase of the cell cycle, the chromosome contains two identical sister chromatids. S- phase is when the DNA replication occurs.
So, a chromosome has one chromatid which is copied in replication.