If two shapes are 'similar', it means that the dimensions of the shapes have the same ratio towards each other. In this example, we can see that the side with a length of 2 correlates to the side with a length of 4. That means, the scaling ratio is 4:2, which can be reduced to 2:1.
The next step is to look at the other side. We know that the width of the smaller shape is 1, and that the second shape is twice the size of the smaller shape (which is what a scale of 2:1 means). Therefore, you just need to multiply the known width by two to get the unknown.
1 * 2 = 2
The missing side _d_ has a length of 2.
Hope that helped =)