Consider the bottom most rectangle.
Its length = 5 - 2 = 3 units
Its width = 3 - 1 = 2 units
So, its area = 3(2) = 6 square units.
Consider the middle rectangle.
Its length = 2 - 1 = 1 unit
Its width = 4 - 2 = 2 units
So, its area = 1(2) = 2 square units.
Finally, consider the top rectangle.
Its length = 6 - 1 = 5 units
Its width = 6 - 4 = 2 units
Its area = 5(2) = 10 square units.
Hence, total area = 6 + 2 + 10 = 18 square units. Correct option is C.
