Cost of acquisition for property converted from tenancy to ownership

Dear All, Need your expert advice on following matter: I had acquired one property around 1970's under tenancy rights. The same was converted to ownership mode in 1999 after paying 100 months rent as per law. This property has been sold in 2015 and I would like to know what can be taken as cost of acquisition for this property for income tax purpose: Option 1: Ready reckoner rate as per Nov 99 and index it by 1081/389 Refer case law of Tauqeer Fatema Rizvi. Option 2: Consider 100 months rent paid as cost of acquisition Option 3: Ready reckoner rate as per 1 Apr 1981 and index it by 1081/100 From official income tax website ​In respect of capital asset acquired before 1st April, 1981 is there any special method to compute cost of acquisition? ​​Generally, cost of acquisition of a capital asset is the cost incurred in acquiring the capital asset. It includes the purchase consideration plus any expenditure incurred exclusively for acquiring the capital asset. However, in respect of capital asset acquired before 1st April, 1981, the cost of acquisition will be higher of the actual cost of acquisition of the asset or fair market value of the asset as on 1st April, 1981. This option is not available in the case of a depreciable asset.​ Thanks & Regards, Naren