Correct Option (b):
The Preamble to the Constitution of India explicitly states the date on which the Constituent Assembly adopted, enacted, and gave to themselves this Constitution. This date is "twenty-sixth day of November, 1949". On this date, the Constitution was formally adopted by the Constituent Assembly. While certain provisions came into force immediately, the full commencement of the Constitution occurred on January 26, 1950, which is celebrated as Republic Day. November 26 is observed as Constitution Day (Samvidhan Divas).
Incorrect Options:
Option (a) twenty-sixth day of January, 1950: This date marks the full commencement of the Constitution of India, when it came into force, not the date of its adoption by the Constituent Assembly. This day is celebrated as Republic Day.
Option (c) Twenty-sixth day of January, 1949: On this date, the Constituent Assembly was still in the process of drafting the Constitution. The final draft was not ready for adoption.
Option (d) None of the above: This option is incorrect because the correct date, twenty-sixth day of November, 1949, is explicitly stated in the Preamble itself and is available among the given choices.