The least number of squares to be added in the figure to make AB a line of symmetry is

For the horizontal line $AB$ to be a line of symmetry, every square on one side of $AB$ must have a corresponding symmetric mirror image on the opposite side of $AB$.

1. Analysis of the given figure:

   • Above line $AB$: There is $1$ square.

   • Below line $AB$: There are $5$ squares.

2. Finding the required additions:

   • The $1$ square above line $AB$ does not have a mirror-image square below $AB$. Thus, $1$ square must be added below $AB$.

   • The $5$ squares below line $AB$ do not have corresponding mirror-image squares above $AB$. Thus, $5$ squares must be added above $AB$.

3. Total squares to be added:

   $$\text{Total squares} = 5 + 1 = 6$$

Therefore, the least number of squares to be added is 6, which corresponds to option (A).