What is the volume (in ${\text{cm}}^3$) of model below formed using cubes of volume $1{\text{cm}}^3$?

1. 20

The correct option is A 20
The volume of the model can be calculated easily using the unit cubes.

There are 2 unit cubes in the top layer of the model, which are marked in green.


There are 9 unit cubes in the second layer, which are marked in blue.

Lastly, there are 9 unit cubes in the lowest layer of the model, which are marked in red.

Thus, there are $2+9+9=20$ unit cubes in all.

The volume of the model equals the sum of volumes of all the unit cubes in it.

Since there are 20 unit cubes, the volume of the model is $20{\text{cm}}^3.$