If you have information in Boxes 1b and 1c, you
will be able to tell which box to mark.
If the date in Box
1b is a more than a year before the date in Box 1c, then the gain or loss is long-term.
If the dates are a
year or less apart, the gain or loss is short-term.
Click
here for more information about Short-term vs Long-term Capital Gains.