Final answer:
Decreasing the shortest whale length from 23m to 20m will cause the mean to decrease as it depends on the sum of all lengths. The median may either decrease or stay the same, but without specific data, it's common that the median remains unchanged in a large data set, leading to option C as the most likely scenario.
Step-by-step explanation:
When analyzing the effect of changing one value in a data set, Diego must consider both the mean and the median. The mean is calculated by adding all the lengths together and dividing by the number of observations. By decreasing the smallest value from 23m to 20m, the total sum of the lengths decreases, which, when divided by the number of whales, will result in a lower mean. Therefore, the mean will decrease.
However, the median, being the middle value when all lengths are ordered from smallest to largest, may or may not change depending on how many data points there are and where the changed value falls in relation to the middle of the data set. If the median was initially the value of 23m that was changed, then the median will decrease. If the initial median was not 23m, and given that all other lengths are between 23m and 27m, the new value of 20m may not affect the position of the middle value, and thus, the median could stay the same.
To give a definite answer, more information about the specific position of the median would be needed; however, the most common scenario in a large data set is option C: the mean will decrease, and the median will stay the same.