When comparing data between two different groups, A.
the middles of the data sets and the spreads around them should both be considered.
B.
the most frequently occurring values and the middle values should both be removed from the data sets.
C.
only the highest and lowest values of the data sets should be considered.
D.
only the average values of the data sets should be considered.

A.  the middles of the data sets and the spreads around them should both be considered.

Explanation:

When comparing data from two different groups, the most frequently occurring values should not be removed because they are most representative of the data and so will be very important in comparing the data.

The highest and the lowest values should not be the only data considered, indeed those should be the of lower importance than the values in the middles as the highest and lowest can sometimes be outliers.

Averages are good for summaries but shouldn't be the only things considered. The correct answer is therefore the first option that the middles and the spreads around the middle should be considered as these are more indicative of the groups.