What is the only reasonable and ethical way to achieve zero population growth

By not having more offspring.

Explanation:

There are many ways of reaching a zero population growth, but some of them are 0% ethical. But, if the population stops having offspring, there won't be a population growth, instead, the population will decrease in number.

In 2017 the country with the biggest population was China, and the country with the biggest population growth in that same year was South Sudan, with a 3,83% population growth, according to statista and IndexMundi Currently, in 2019, China still is the country with the biggest population, and, as for the highest population growth this year, we will only be able to say with 100% certainty when the year ends.

Hope it helped,

BiologiaMagister