The Foreign Institutional Investors (FIIs) were net buyers to the tune of Rs 591.45 crore in the futures & option segment on Thursday.According to data released by the NSE, FIIs were net buyers of index futures to the tune of Rs 425.29 crore and bought index options worth Rs 278.15 crore. They were net sellers of stock futures to the tune of Rs 114.95 crore while bought stock options worth Rs 2.96 crore.