The correct answer is option 2: Azerbaijan.

ONGC Videsh, which is the state-owned oil and gas company of India, signed a definitive agreement with Azerbaijan in September 2012. It is important to note that a definitive agreement is a legally binding contract that outlines the terms and conditions of a partnership or collaboration.

Option 1: Russia is not the correct answer. While ONGC Videsh may have signed agreements with Russia in the past, the specific agreement mentioned in the question is with Azerbaijan.

Option 3: Iran is also not the correct answer. While ONGC Videsh may have collaborations or agreements with Iran, the question specifies that the agreement was signed with Azerbaijan.

Option 4: South Sudan is not the correct answer. ONGC Videsh may have engagements with South Sudan, but the agreement mentioned in the question is with Azerbaijan.

Therefore, the correct answer is option 2: Azerbaijan.