In which one of the following groups are all the four countries members of G20?

The correct option is Argentina, Mexico, South Africa and Turkey.

To understand why this is the correct group, let us review the complete list of G20 members. The G20 (Group of Twenty) is a premier forum for international economic cooperation. It comprises 19 individual countries, the European Union, and the African Union.
The 19 member countries are:

1. Argentina
2. Australia
3. Brazil
4. Canada
5. China
6. France
7. Germany
8. India
9. Indonesia
10. Italy
11. Japan
12. Mexico
13. Russia
14. Saudi Arabia
15. South Africa
16. South Korea
17. Turkey (Türkiye)
18. United Kingdom
19. United States

Now, let us analyze each of the given options to see why they are correct or incorrect:

1. Argentina, Mexico, South Africa and Turkey:
Checking against the member list:
- Argentina is a member.
- Mexico is a member.
- South Africa is a member.
- Turkey is a member.
Since all four countries in this group are members, this is the correct option.

2. Australia, Canada, Malaysia and New Zealand:
- Australia and Canada are G20 members.
- However, Malaysia and New Zealand are not members of the G20.

3. Brazil, Iran, Saudi Arabia and Vietnam:
- Brazil and Saudi Arabia are G20 members.
- However, Iran and Vietnam are not members of the G20.

4. Indonesia, Japan, Singapore and South Korea:
- Indonesia, Japan, and South Korea are G20 members.
- However, Singapore is not a member of the G20 (although it is frequently invited as a guest country).