The Foreign Institutional Investors (FIIs) were net sellers to the tune of Rs 512.55 crore in the futures & option segment on Thursday.According to data released by the NSE, FIIs were net sellers of index futures to the tune of Rs 497.23 crore and sold index options worth Rs 165.69 crore. They were net buyers of stock futures to the tune of Rs 152.23 crore while sold stock options worth Rs 1.86 crore.