Assertion: Two to three days after whitewashing walls become shining.
Reason: Calcium hydrogen carbonate is formed when calcium hydroxide reacts with carbon dioxide.

In whitewash, calcium hydroxide is present. A thin layer of calcium carbonate is formed on the walls as a result of its gradual reaction with atmospheric carbon dioxide. After two to three days of whitewashing, calcium carbonate is produced. Thus, the brilliant white finish on the walls shows after two to three days.
Thus, the assertion is true, but the reasoning is false as calcium carbonate is formed, not calcium hydrogen carbonate.
Hence, the correct option is c) $A$ is true, but $R$ is false.