Banking stock Citigroup Inc. (NYSE:C - 39.49) has attracted the attention of option bulls lately. During the past five days, traders on the International Securities Exchange (ISE) and Chicago Board Options Exchange (CBOE) have bought to open 48,895 calls on C, compared to only 18,011 puts -- netting the shares a call/put volume ratio of 2.71, with bullish bets nearly tripling their bearish counterparts.

From a broader perspective, C now sports a 10-day call/put volume ratio of 2.17, based on cumulative buy-to-open volume from the ISE, CBOE, and NASDAQ OMX PHLX (PHLX). This ratio ranks higher than 86% of other such readings taken over the past 52 weeks, suggesting speculative players have purchased calls over puts at a faster clip only 14% of the time during the last year.

A preference for calls is also reflected by C's Schaeffer's put/call open interest ratio (SOIR) of 0.41. Not only does this ratio indicate that calls more than double puts among options set to expire within three months, it also ranks in just the 7th percentile of its annual range. In other words, short-term speculators have been more call-heavy on the stock just 7% of the time during the past year.

In the newly front-month January 2013 series, heavy call open interest rests overhead at the 40 strike. Currently, C's January 40 call boasts 114,932 contracts in residence -- all of which are out of the money by less than one point. During the short term, this glut of overhead calls could act as a temporary speed bump for the stock.

In light of the bank's positive price action in 2012, the trend toward C calls over puts isn't too surprising. The stock has gained more than 50% year-to-date, putting the broader equities market to shame. Plus, a recent pullback from the aforementioned round-number $40 region was neatly contained by the equity's rising 10-day moving average.