The mode of the data shown on the stem-and-leaf plot is

To solve this problem you must apply the proccedure shown below:
 1. By definition, the mode is the value that appears the most.
 2. Keeping the definition of the mode on mind, you can see in the stem-and-leaf plot that the value that appears the most is 5.1 (three times)
 3. Therefore, you can conclude that the mode of the data shown on the stem-and-leaf plot is 5.1.
 The answer is: 5.1