The 73rd Constitutional Amendment (1992, effective 1993) granted constitutional status to Panchayati Raj institutions including the Gram Sabha - A is correct. The Balwant Rai Mehta Committee (1957) did recommend the three-tier system mentioning Gram Sabha - R is also correct. However, the 73rd Amendment was influenced mainly by the L.M. Singhvi Committee, so R does not directly explain A.