Concept:
The Constitution of India permits preventive detention even during normal times. It is not dependent on the proclamation of an emergency. Preventive detention laws are enacted to prevent a person from acting in a manner prejudicial to the security of the State, public order or other specified interests.
Step 1: Recall the Constitutional provision.
Preventive detention is recognized under Article 22 of the Constitution of India and may operate both during peace and war.
Step 2: Analyse the options.
• At any time during peace or war irrespective of emergency -- Correct.
• Only during internal emergency -- Incorrect.
• Only during national emergency -- Incorrect.
• Only when India is at war -- Incorrect.
Step 3: Final conclusion.
Hence, preventive detention can be exercised
\[
\boxed{\text{At any time during peace or war irrespective of whether an emergency has been proclaimed}.}
\]