Step 1: Understanding the Concept:
The Planning Commission was a body that decided India's five-year plans from 1950 onwards. The government later felt this top-down model did not fit a fast-changing economy, so it was replaced by a new body meant to work more like a think tank.
Step 2: Fixing the date.
NITI Aayog (National Institution for Transforming India) was set up on 1 January 2015 through a Cabinet resolution, replacing the Planning Commission.
Step 3: Why the other options are wrong.
2016, 2017, and 2019 are all years after NITI Aayog was already functioning, marking later stages of its work, not its creation.
Final Answer:
NITI Aayog was established in 2015. This corresponds to option (A).