What substance has the European Union banned from nail polish due to its toxicity in reproduction for animals?

The substance that has been banned by the European Union from nail polish due to its reproductive toxicity in animals is TPO, also known as triphenyl phosphate. The decision to ban TPO stems from research indicating that it poses significant risks to reproductive health and can interfere with endocrine functions. The EU prioritizes the protection of health and the environment, which leads to stringent regulations on chemicals that are deemed hazardous.

This ban aims to mitigate potential risks to human health and safeguard environmental standards. In contrast to TPO, other substances listed like formaldehyde, acetone, and phthalates have different regulatory environments or are subject to restrictions but are not the specific focus of the EU's nail polish ban in this context. Therefore, TPO is recognized as the correct answer for its specific association with reproductive toxicity concerns leading to its prohibition in nail products within the EU.