1. Your layman's instinct is appreceable while deriving that it should be STCG since you became the title holder of the property in the year 2013 and sold it in the same year,
2. Had you not registered the prperty in the year 2012, which made you its title holder, you would not have been in a legal position to sale and register the sale deed in the year 2013,
3. However, while calculating the capital gain tax, the argument is different. You have invested the full amount of the consuderation in the year 2000 and had taken possession of the flat after which you made capital gain in the year 2013 when you sold that flat,
4. So, your investment on the flat in the year 2010 fetched you a capital gain in the year 2013 i.e. after a span of 13 years,
5. Based on the above logic the instant gain is LTC Gain.