Assertion : $\mathrm{BaCO}_3$ is more soluble in $\mathrm{HNO}_3$ than in plain water. Reason : Carbonate is a weak base and reacts with the $\mathrm{H}^{+}$from the strong acid, causing the barium salt to dissociate.

• Correct answer
  A

  If both assertion and reason are true and the reason is the correct explanation of the assertion
• B

  If both assertion and reason are true and the reason is not the correct explanation of the assertion
• C

  If assertion is true but reason is false
• D

  If both assertion and reason are false.

Hence, the correct answer is option: 'A'.