The Foreign Institutional Investors (FIIs) were net buyers to the tune of Rs 839.44 crore in the futures & option segment on Tuesday.According to data released by the NSE, FIIs were net buyers of index futures to the tune of Rs 327.10 crore and bought index options worth Rs 583.10 crore. They were net sellers of stock futures to the tune of Rs 66.56 crore and sold stock options worth Rs 4.19 crore.