The detail that Mateo might take out of his essay about unique animals could vary depending on the context or focus of his essay. However, if we assume he is trying to emphasize only truly unique characteristics that set certain animals apart significantly, he might choose to remove:
B. All mammals give birth to live young.
This is because it's a general characteristic of mammals rather than a unique trait about a specific animal. The other options highlight unique or distinctive features of particular species.