Sister chromatids are two identical copies of a single chromosome that are connected by a centromere. They occur as a result of a chromosome that duplicated during the S phase of the cell cycle.
The monochloroderivatives will be obtained by substituting chemically non equivalent hydrogen with chlorine atom, one by one
So the possible monochloro derivatives of 2,4-dimethylpentane (figure 1) are shown in figure (2)
The answer is rubidium -91 because it takes a shorter time of 58.4 seconds to become stable.