To determine which significant initiative or event took place during Dr. Manmohan Singh's tenure as the Prime Minister of India (2004–2014), we need to analyze the given options and understand their historical context:
Based on the above analysis, the correct answer is the Implementation of the National Rural Employment Guarantee Act (NREGA), as it was indeed implemented during Dr. Manmohan Singh's tenure as the Prime Minister of India (2004–2014).