Indexed cost of acquisition

You have to calculate indexation separately for land and building

for land take the CII in the year of purchase

for building take the CII in the year of completion of construction

Cost of acquisition indexation shall be taken CII of the year of purchase.

CII of the year of sale shall also be taken

Thank you sir but what if there is no completion certificate available is proof of completion required to be furnished???

Then you have to take the CII of the year of completion

It is not a problem if you dont have completion certificate.
you may furnish other details like first utility bills or first property tax bill.

if you dont have any on these then your memory will best serve the purpose. you may select the year which you remember as the year of completion.
it is advisable to keep contact details of the construction contractor who could affirm on such claim of year of cometion.

the burden on prooving will arise only when AO takes your matter for consideration. else, things go very smoothly for most people

Fully accepted @ veer's reply.