A) MgCO₃ is insoluble
Based on the rules of solubility:
All nitrates are soluble. That means Pb(NO₃)₂ is soluble.
All ammonium ions are soluble. NH₄Br is soluble.
All phosphate are insoluble except when combined with group I ions (Li, Na, K...) K₃PO₄ is soluble.
Also, all carbonates are insoluble except when combined with group I ions. That means Li₂CO₃ is soluble and:A) MgCO₃ is insoluble