Which age structure diagram is better ( pyramidal or zero population growth?

"Zero Population Growth" is a calculable balance between births and deaths in a population; a pyramidal age structure diagram is a mapping of demographics to look for trends in a population. These two concepts are entirely different.

A real population condition has nothing to do with the second theoretical value, which is only useful for projections. The first is used for observation and study of past trends and current states.

For the majority of environmental studies, the pyramidal age structure will be most helpful.

The age structure diagram that indicates zero population growth is generally considered better for sustainability and stability, as it suggests a balanced population with similar numbers in younger and older age groups. This indicates a more sustainable population growth pattern compared to a pyramidal age structure, which typically indicates high birth rates and a younger population, potentially leading to population growth challenges in the future.

The suitability of an age structure diagram depends on various factors such as population growth goals, economic conditions, and social policies of a particular region or country. There isn't a universal "better" diagram; rather, it depends on the specific circumstances and objectives of the population in question.