How long dated options can we buy on Zerodha ? Can we buy or sell Dec 2021 expiry ?
Not possible… Because options contact have 3 months trading cycle in India…
That’s near Month( month 1), next month ( month 2) , far month (month 3) contacts.
New option contracts are introduced on the next trading day of the expiration of the monthly contracts…so at any point of time a trader has a option to choose from the 3 contacts available…
Am i right sir @ShubhS9 
Thats only true for stock options. For the index you can do LEAPS because the NSE website shows the option chain right up to 2024 and some full service brokers allow you to trade contracts way into the future provided there is liquidity
You can, given there is liquidity. You can search for these Options on Kite, for December 2021 Option type "Nifty 21DEC -Strike Price-.