Looking at options trading activity among components of the S&P 500 index, there is noteworthy activity today in Merck & Co Inc (MRK), where a total volume of 53,171 contracts has been traded thus far today, a contract volume which is representative of approximately 5.3 million underlying shares (given that every 1 contract represents 100 underlying shares). That number works out to 50.3% of MRK's average daily trading volume over the past month, of 10.6 million shares..