What is a common application of formaldehyde outside of cosmetics?

Formaldehyde is widely recognized for its utility in preserving biological specimens. This application is particularly crucial in fields such as medicine and biology, where it is used to maintain tissue samples, organ specimens, and other biological materials for research, education, and diagnostic purposes. Its effectiveness in preventing decay and maintaining the integrity of biological samples makes it invaluable for preserving specimens at room temperature, which might otherwise decompose or change over time.

In contrast, the other options do not accurately represent common applications of formaldehyde. It is not used as an anesthetic in surgical procedures, nor is it incorporated in the creation of fragrances for skin care products. Also, formaldehyde is not recognized as a natural dietary supplement; in fact, exposure to formaldehyde can be hazardous to health. Therefore, the application of formaldehyde in preserving biological specimens stands out as its most relevant and widely acknowledged use outside of cosmetics.