Butterflies laying their eggs on milkweed plants occurs at what level of the biosphere hierarchy?

The correct answer would be a community.

A biological hierarchy can be written as cells → tissues → organs → organ systems → organisms → populations → communities →ecosystem → biosphere.

The community is defined as the interactive populations of two or more species present in the same geographical area at a particular time.

Here, butterflies and the milkweed plants interact in such as way that the plants provide nectar and space or nurseries to the butterflies and in return, butterflies help in the pollination of these plants and thus helps in the reproduction process.  

Thus, they interact with each other in the same geographical area which comes under the community level of the biosphere hierarchy.

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a. the production of carbon dioxide through the process of photosynthesis