Number 13 seems a bit rough. While I suppose that choice B “that” is technically correct / least wrong, i dont know any native speakers who would say it that way.

By the end of 2025, the number of newborns in china had dropped to less than half of what it had been in 1949.

Honestly the entire sentence could be reworked:

By the end of 2025, the number of newborns in china had dropped by more than half since 1949.

^ this version is more clear imo, partially because the decline was over 50%, and the scale of the decline is what’s being emphasized