The correct option is C 20
The total number of layers of cubes is 4.
Total number of cubes in the top layer = 1
Total number of cubes in the second layer = 3 (1 is hidden)
Total number of cubes in the third layer = 6 (3 are hidden)
Total number of cubes in the lower layer = 10 (6 is hidden)
So total number of cubes
= 1 + 3 + 6 + 10 = 20.