Answer: B. 36 feet.
Number of strings = 8
Explanation:
From , the given line plot, the number of strings used by Mario = Number of cross on the line plot
i.e. Number of strings used by Mario = 8
Now by the given line plot , the sum of all the observations will be ( multiply all values by their corresponding frequencies or crosses and add ) :-

Now, the total length of ribbon used by Mario = Sum of all the observations
Hence, the total length of ribbon used by Mario 36 feet.