Question

he has been running a clandestine operation for the past 10 years, passed down to him by a professor when he was a medical student. source: national geographic in the sentence above, clandestine has the same or almost the same meaning as: clinical regional costly secret

Explanation:

The word "clandestine" means kept secret or done secretively. "Secret" has a similar meaning. "Clinical" relates to a medical setting, "regional" to an area, and "costly" to expense, none of which are similar to "clandestine".

Answer:

D. secret