As of 2021, which of the following is the longest expressway of India, that has 6 lanes, likely to be…

The correct choice is: Purvanchal Expressway

The Purvanchal Expressway in Uttar Pradesh, from Lucknow to Ghazipur, is about 341 km long, six lanes wide (expandable to eight) and was opened in November 2021. At that time it was India's longest expressway. The Yamuna Expressway (about 165 km) opened back in 2012, the Delhi–Meerut Expressway is under 100 km, and the Himalayan Expressway is a short road to Parwanoo.

Hence the answer is Purvanchal Expressway.

Now: longer expressways have since opened, e.g. the 701 km Mumbai–Nagpur (Samruddhi) Expressway, fully open since June 2025, while the ~1,350 km Delhi–Mumbai Expressway is being opened in phases.