The foreign institutional investors (FIIs) were net buyers to the tune of Rs 535.53 crore in the futures & option segment on Monday.According to data released by the NSE, FIIs were net buyers of index futures to the tune of Rs 478.19 crore and they bought index options worth Rs 100.66 crore. They were net sellers of stock futures to the tune of Rs 61.44 crore while they bought stock options worth Rs 18.12 crore.