The correct answer to this open question is the following.
Although there are no options attached we can say the following.
Why is the agricultural workforce an indicator of low-level development?
Because in developing countries there is not a large industry because governments do not have the capacity or the programs to support the development of a solid industry and the technology to help the country grow.
Private companies find it difficult to invest in industries, and there are just a few that work in these countries.
That is why in these kinds of countries, agriculture plays a major role in the economy, because many developing countries have enough raw materials and natural resources, but no money to build industries to transform those raw materials and natural resources into goods to export and sell.
So the economy is based on agriculture, the export of cash crops, and the internal consumption of those crops to feed the people.