Question 1020308
a.  Either the median or the mean.  The median has the advantage of being less subject to extreme values, but the mean has the advantage of pointing out those extreme values.  Still, the median would be more useful, as it would describe upper and lower halves.
b.  Blood type: Mode.  Mean makes no sense, nor does median.  Want to know the numbers for each type.  Even that is limited in value.
c.  Rankings:  Mode for the same reason.  Want to know numbers for each.  The mean has no useful value, and the median might be useful to show where half are ranked above/below, but neither is as good as the mode.