The foreign institutional investors (FIIs) were net buyers to the tune of Rs 684.71 crore in the futures & option segment on Thursday.According to data released on the NSE website, FIIs were net buyers of index futures to the tune of Rs 193.72 crore while they bought index options worth Rs 51.58 crore. They were net buyers of stock futures to the tune of Rs 439.41 crore.