Concept:The constricted region of a chromosome, where sister chromatids remain attached, is called the centromere.
Explanation:During cell division, a chromosome appears as two identical sister chromatids joined at a narrow region.
The part labelled I points to this narrow region.
Spindle fibres attach to this region to pull chromatids apart.
This region is not the nucleolus, which is inside the nucleus.
It is not a centriole, which forms spindle fibres.
It is also not the spindle itself, which is made of protein fibres.
Therefore, the labelled structure is the centromere.
Answer:B. centromere
