Devaluation of the domestic currency makes exports cheaper and imports more expensive, which increases demand for domestically produced goods. This boosts production, employment, and, ultimately, the national income, making Assertion (A) true.
Reason (R) correctly defines devaluation under a fixed exchange rate system, making it true as well. However, Reason (R) merely describes devaluation and does not explicitly explain its impact on national income, making it not the correct explanation for Assertion (A).