Consider the following countries:

I. Austria

II. Bulgaria

III. Croatia

IV. Serbia

V. Sweden

VI. North Macedonia

How many of the above are members of the North Atlantic Treaty Organisation?

The correct option is Only four.

To determine how many of the given countries are members of the North Atlantic Treaty Organisation (NATO), we can examine the membership status of each country listed:
I. Austria: Austria is a neutral country and is not a member of NATO.
II. Bulgaria: Bulgaria joined NATO in the year 2004.
III. Croatia: Croatia joined NATO in the year 2009.
IV. Serbia: Serbia is militarily neutral and is not a member of NATO.
V. Sweden: Sweden officially joined NATO in the year 2024.
VI. North Macedonia: North Macedonia joined NATO in the year 2020.

Counting the member states from the list:
1. Bulgaria (Member)
2. Croatia (Member)
3. Sweden (Member)
4. North Macedonia (Member)

Therefore, exactly four of the six listed countries are members of NATO.