Best Option is to sell their infrastructure to Jio (Fiber and Cellular) and Airtel (Fiber and Cellular) and then close-down the business operations. (Provide backend support to the Airtel/Jio as per their needs until the deal is done 100%).
Reason: Jio itself would control the largest fiber network of the country if they buy the major stakes in BSNL. That's why, selling a appropriate amount to both Jio and Airtel (divided regionally) would be the best bet.
Airtel can take the North and East and Jio must take South and West.
Airtel has good expansion in the North (Delhi, Gurgaon, Noida and Punjab) whereas Jio has their major fiber subscriber in the south (Mumbai).
P.S.: Even though they have clarified that they aren't going to close down. But just in case if they plan to do so. They should sell themselves to Airtel and Jio instead of selling any one of them.