Nature of capita gain stcg or ltcg

My father had booked a flat tunder lottery system at Kolkata in the year 2008 and has been received the Provisional allotment in the same year. However Flat was ready for possession in the year 2014 . Cost of the flat was Rs. 6.53 Lacs under Installment payment system. Till 2010, he had already paid Rs. 6.30 Lacs on account of appx. 95% of cost and balance Rs. 33000/-along with other charges and Parking fees, registration etc. (appx. 1.47 lacs) was paid by him in the year 2014 when he received Possession of the flat and Letter of possession received in Oct, 14whereas registration of deed was done in March, 2015. Now he wants to sell the property at around 20 lacs resulting capital gain of Rs. Appx. 12 lacs. Now I am confused that whether it will be treated as Short Term Capital Gain or Long Term Capital Gain. Out of total 8 lacs, he had paid 6.20 lacs under instalment before 4 to 7 years but since the Possession of the flat and registration completed recently, whether he will get the benefit of LTCG Taxes(20% taxes) and indexation benefit or liable to pay tax at normal rates under STCG. Please advice. Regards, Amit