A line of symmetry is defined as a line that divides a figure into two parts in a way such that each part is a mirror image of the other part about that line.
The given figure consists of $16$ unit squares arranged as shown. In addition to the three black squares, what is the minimum number of squares that must be coloured black, such that both $\text{PQ}$ and $\text{MN}$ form lines of symmetry? (The figure is representative)
- $3$
- $4$
- $5$
- $6$