Final answer:
The average adult cell turnover rate is every 30 days (B) for a variety of cell types, although the rate can vary from a few days for some cells to a lifetime for others. The correct option is B.
Step-by-step explanation:
The average adult cell turnover rate varies significantly depending on the type of cell. Epithelial cells have a rapid turnover time, averaging between two to five days. However, some specialized cells like cortical neurons or cardiac muscle cells can last a lifetime without dividing, existing in what is known as the Go phase of the cell cycle.
Therefore, the correct answer to the student's question is B) Every 30 days, as this can be seen as a generalized average considering various cell types, despite some cells renewing more frequently and others much less so.