How is the median defined if the number of observations is even? a. the average of the two middle observations b. the difference between the two middle observations c. the most frequent observation d. the difference between the highest and smallest observation?

The answer is A.
In getting the median if there's an even number of observations, first, you need to arrange the data from least to greatest (or greatest to least will do).
Then, add the two middle observations, then divide the answer by 2, to get the average of the two middle observations. The answer you will get from dividing it by two is the median.