Final answer:
The final rule signed on October 1, 2015, went into effect last on December 28, 2015, marking the transition from the 2008 to the 2015 standards.
Step-by-step explanation:
The final part of the regulation that went into effect was the final rule signed on October 1, 2015, which became effective on December 28, 2015. This rule was part of a transition from previous standards set in 2008 to the current standards established in 2015.
The process of revocation of the 2008 standards in favor of the new 2015 standards will occur as outlined in the implementation rule for these current standards. Importantly, this changeover represents a significant update and refinement in regulatory practices which would have wide-reaching implications for those governed by such regulations.
The part of the regulation that went into effect last is the current (2015) standards. The previous (2008) 03 standards are still in effect in some areas. The implementation rule for the current standards will address the revocation of the previous standards and the transition to the current standards.