D) 15
Explanation:
Ignore the stems. We count out the number of leaves
row one: 3 leaves (3,7,7)row two: 5 leaves (2,4,4,4,5)row three: 4 leaves (1,5,8,9)row four: 3 leaves (0,6,7)
So we have a total of 3+5+4+3 = 15 leaves which correspond to 15 data values.