Scientists consider the testing phase of the cell cycle because they can’t see a visible change during this phase

Scientists consider the INTERPHASE the resting phase of the cell cycle because they can not see a visible change during this phase.
The interphase refers to the interval in the cell cycle between two cell divisions. During this stage, the individual chromosomes in the cell can not be distinguished. Although scientist can not see any outward visible change during this phase, but it is actually the time when the DNA inside the nucleus is replicated.

The answer is c : ) hope this answered your question!