On 26 November 2008, in the aftermath of the terrorist attacks in Mumbai, at the national level, which investigative agency was established?

The question asks: "On 26 November 2008, in the aftermath of the terrorist attacks in Mumbai, at the national level, ....... this investigative agency was established."

Let's analyze the options:

• 1) दहशतवाद विरोधी पथक (Anti-Terrorism Squad - ATS): These are state-level units, not a national investigative agency established specifically after 26/11 for that purpose.
• 2) केंद्रीय अन्वेषण ब्युरो (Central Bureau of Investigation - CBI): The CBI is a national investigative agency, but it was established much earlier and has a broader scope, not solely focused on terrorism and not created in response to the 26/11 attacks.
• 3) फोर्स वन (Force One): This is a specialized counter-terrorism unit of the Maharashtra Police, meaning it is a state-level force, not a national investigative agency.
• 4) नॅशनल इन्वेस्टीगेशन एजन्सी (National Investigation Agency - NIA): The National Investigation Agency (NIA) was established by the Indian Government in 2008, specifically in the wake of the 26/11 Mumbai terror attacks, to combat terror-related crimes across the country. It is a national-level investigative agency.

Based on this, the correct agency established at the national level after the 26/11 Mumbai attacks is the National Investigation Agency.

The correct option is: 4) नॅशनल इन्वेस्टीगेशन एजन्सी

The final answer is $\text{4}$