The conclusion that can be safely drawn is that "The stains could be used to identify mitochondria in living cells"
Explanation:
Mitochondria are the powerhouse of the cells. It produces energy molecules (ATP molecules) to power various cell functions. Hence mitochondria are associated with living cells.
As provided biologist treated some cells with stains and later she found that these were concentrated in the power-house of the cells.
Since the Biologist found that stains were concentrated within the mitochondria of many of the cells, hence it can be safely argued that stains(which are present in the mitochondria) can be used to confirm the mitochondria in those of living cells.