Weakest base among $\mathrm{KOH},\mathrm{NaOH},\mathrm{Ca}(\mathrm{OH}{)}_2$ and $\mathrm{Zn}(\mathrm{OH}{)}_2$ is:

$\mathrm{KOH},\mathrm{NaOH},$, and $\mathrm{Ca}(\mathrm{OH}{)}_2$ are examples of strong bases since they are s-block elements. It is a weak basic, Zn(OH)2. It occasionally also exhibits amphoteric base behavior. The weakest base among the offered hydroxides is $\mathrm{Zn}(\mathrm{OH}{)}_2$
The following hydroxides are listed in decreasing order of basicity:

$\mathrm{KOH}>\mathrm{NaOH}>\mathrm{Ca}(\mathrm{OH}{)}_2>\mathrm{Zn}(\mathrm{OH}{)}_2$
$\mathrm{Zn}(\mathrm{OH}{)}_2$ is therefore the least stable base of the alternatives.