A protein complex, that links sister chromatids, is

The correct option is cohesin.

Cohesin is a highly conserved multi-subunit protein complex that plays a critical role in chromosome segregation during cell division. Its primary function is to establish and maintain physical cohesion between sister chromatids from the time of DNA replication in the S phase of the cell cycle until their segregation during mitosis or meiosis.

Here is a step-by-step breakdown of how cohesin functions and why it is the correct answer:

1. Establishing Connection: During DNA replication (S phase), the DNA is duplicated to produce two identical sister chromatids. The cohesin complex forms a ring-like structure that physically encircles and holds these sister chromatids together.
2. Aligning Chromosomes: In metaphase, the sister chromatids align along the metaphase plate of the cell. Cohesin resists the pulling forces exerted by the spindle microtubules, establishing the tension necessary to signal that the chromosomes are correctly attached and ready for segregation.
3. Anaphase Onset: Once all chromosomes are properly aligned, the enzyme separase is activated to cleave cohesin. This allows the sister chromatids to separate and move to opposite poles of the dividing cell.

Analysis of Other Options:

- Actin is a structural protein that polymerizes to form microfilaments. It is key to maintaining cell shape and forming the contractile ring during cytokinesis, but it does not link sister chromatids.
- Myosin is a motor protein that interacts with actin to produce force and movement, primarily in muscle contraction and cellular transport.
- Actomyosin refers to the contractile complex formed by actin and myosin, which is responsible for contractile forces during cell movement and cytokinesis.