Which of the following salt solutions will have lowest pH value? 

In ${\mathrm{CaCO}}_3$, both ions show hydrolysis. In ${\mathrm{CaCl}}_2$, only ${\mathrm{Ca}}^{2+}$ is hydrolyzed. Both $\mathrm{Ca}(\mathrm{OH}{)}_2$ and ${\mathrm{CH}}_3\mathrm{COONa}$ will be alkaline in nature. Hence, the lowest pH will be that of ${\mathrm{CaCl}}_2$ solution.