"cell biologists have introduced radioactively labeled carbohydrates, fats and amino acids to living cells, allowed time for cell metabolism, and then separated the various cell organelles from the cytoplasm. the radioactive molecules were detected as part of various cellular compounds, however the visible cell structures appeared unchanged. this leads us to conclude that":

The correct answer will be- the cell structures undergo repeated breakdown and formation.

Explanation:

The carbohydrates, fatty acids and amino acids are the three important bio-molecules which form the components of various cellular structures.

In the given question, the cell biologist radiolabeled these biomolecules and then observed the cellular structures after cell metabolism. He found that few cellular structures contained radiolabeled structures and few not. This showed that the cellular structures constantly undergo changes in their structures due to cellular metabolic reactions and are broken down and reformed.

Thus,  the cell structures undergo repeated breakdown and formation is the correct answer.