You have to determine how you are making the comparison to find out which one is the selected option...
If you are posting the form back to itself, for example, then you'd do something like this:
echo "<select name=category><OPTION VALUE=0>Choose</option>";
while ($row=mysql_fetch_array($result))
{
echo "<OPTION VALUE='" . $row["category"] . "' ";
if (isset($_POST['category']) && $row['category'] == $_POST['category']) // ur logic to find that out
echo " selected ";
echo " >" . $category . "</option>";
}
Without seeing more code, all of the help we offer is just speculation based on our perceptions of what we think your code should be. Post what you have, along with an explanation of what kind of comparison you want to make, and we can help from there.