The correct answer is option 3: Serum Institute of India. The Drugs Controller General of India approved the Serum Institute of India for conducting human trials of the Oxford-AstraZeneca Covid-19 vaccine candidate.

Let`s examine the other options:

Option 1: Bharat Biotech - Bharat Biotech is an Indian biotechnology company. While it has been involved in the development of a Covid-19 vaccine called Covaxin, it was not approved for conducting human trials of the Oxford-AstraZeneca vaccine candidate.

Option 2: AIIMS (All India Institute of Medical Sciences) - AIIMS is a prestigious medical institute in India. While it may be involved in the clinical trials of various medicines, it was not specifically approved for the human trials of the Oxford-AstraZeneca Covid-19 vaccine candidate.

Option 4: National Institute of Epidemiology - The National Institute of Epidemiology is a research institute that focuses on epidemiology and public health. While it may be involved in the evaluation of vaccines and conducting clinical trials, it was not specifically approved for the human trials of the Oxford-AstraZeneca Covid-19 vaccine candidate.

Therefore, the correct answer remains option 3: Serum Institute of India.