Final answer:
The subject of this question is protein purification.
Step-by-step explanation:
c) Protein Purification
In this question, the term 'elution' is used, which is commonly associated with protein purification techniques. Elution is the process of removing a substance from a separation medium, such as a chromatography column. In this case, ferritin is assumed to be eluted at the interstitial volume, which is the volume of the mobile phase that fills the spaces between the stationary phase particles.
This suggests that the student is referring to a specific step in a protein purification procedure. Therefore, the correct answer is protein purification.