Answer:
Exterior of the cell.
Step-by-step explanation:
The pulse chase experiment was done to understand the secretion pathway of the protein at different time intervals. The protein travels through the endomembrane system for its secretion.
The cells of the pancreas are labelled with the leucine which is radioactive in nature. The black spots will be shown on the cell exterior as three hours are enough for the secretion of the protein outside the cell. This indicates that the protein has been secreted out of the cell.
Thus, the answer is exterior of the cell.