Which electrolyte is least effective in causing coagulation of $+\mathrm{ve}$ ferric hydroxide sol?

The power of an ion to cause coagulation increases with its charge. Ferric hydroxide is a positively charged sol that will coagulate when exposed to an anion. 

$$\begin{gathered} {\mathrm{K}}_3\left[\mathrm{Fe}(\mathrm{CN}{)}_6\right]\to 3{ \mathrm{K}}^{+}+{\left[\mathrm{Fe}(\mathrm{CN}{)}_6\right]}^{3-} \\ {\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\to 2{ \mathrm{K}}^{+}+{\mathrm{Cr}}_2{\mathrm{O}}_7^{2-} \\ \mathrm{KBr}\to {\mathrm{K}}^{+}+{\mathrm{Br}}^{-} \\ {\mathrm{K}}_2{\mathrm{SO}}_4\to 2{ \mathrm{K}}^{+}+{\mathrm{SO}}_4^{2-} \end{gathered}$$

In $\mathrm{KBr}$, Bromine has the least negative charge of the options. 

Hence the correct option is (A).