The figure below shows 12 identical squares. What is the least number of such squares that must be added to the figure so that the line $AB$ becomes a line of symmetry?

1. Identify the reflection of each column across the line of symmetry $AB$.

For the line $AB$ to be a line of symmetry, every square above $AB$ must have a corresponding square below $AB$ at the same distance, and vice versa.

• Row U1 reflects to Row D1.
• Row U2 reflects to Row D2.

2. Determine the missing squares in each column.

• Column 1: Has a square at U1, so it needs a square at D1 ($1$ square needed).
• Column 2: Has squares at U2, U1, D1, D2. It is already symmetric.
• Column 3: Has squares at U2, U1, D1, D2. It is already symmetric.
• Column 4: Has squares at U1, D1. It is already symmetric.
• Column 5: Has a square at D1, so it needs a square at U1 ($1$ square needed).

3. Calculate the total number of squares to be added.

Total squares to add $= 1 + 1 = 2$.