The best explanation for slowing population growth in China is:
China’s one-child policy has led to an aging population and an imbalanced sex ratio.
The one-child policy, which was implemented in 1979 and relaxed in 2015, contributed significantly to the slowing population growth by limiting the number of children families could have. This has led to an aging population, as there are fewer young people to balance out the age demographics, and has also resulted in an imbalanced sex ratio due to cultural preferences for male children.