(D) Constitution Adoption and Signing: India's Constitution was adopted and signed by the Constituent Assembly on November 26, 1949, signifying a crucial implementation milestone. (A) Drafting Committee Formation: This event occurred significantly prior to the specified date, rendering it an incorrect choice. (B) Constituent Assembly Commencement: The Constituent Assembly's proceedings began well before November 26, 1949, making this option invalid. (C) National Flag Design: India's National Flag was adopted on July 22, 1947, thus this option is also incorrect.