Step 1: Understand the question.
We must find the exact date the Bharatiya Nagarik Suraksha Sanhita, 2023 (BNSS) came into force, replacing the Code of Criminal Procedure, 1973.
Step 2: Know what the BNSS is.
The BNSS is one of three big new criminal laws. The other two are the Bharatiya Nyaya Sanhita, 2023 and the Bharatiya Sakshya Adhiniyam, 2023. The BNSS replaces the old CrPC.
Step 3: Understand assent and commencement.
The BNSS got Presidential assent in December 2023. But getting assent does not make a law operate at once. The government must notify a separate date when it actually starts.
Step 4: Fix the commencement date.
The government notified 1 July 2024 as the date the BNSS came into force across India. From that date the old CrPC stopped applying to new matters.
Step 5: See what changed from that date.
Criminal investigations and procedures began to follow the BNSS, and several technology-based reforms started working.
Step 6: Check the options and answer.
January 1, 2024 was not the notified date. August 15, 2023 is even before the assent. December 25, 2023 is not the enforcement date. So July 1, 2024 is correct.
\[ \boxed{\text{July 1, 2024}} \]