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.
Long term capital gain shall be calculated by deducting index cost of acquisition and indexed cost of improvement from net sale consideration.
In the given case the capital gain will be long term capital gain and taxable in the hands of partnership firm.
For tax saving purpose you can invest in specified securities( Bonds of NHAI, REC) under section 54EC upto rs 50. You can also avail the benefit of section 54GB.