Which solution contains 5 mL of formic acid and 90 mL of formalin?

The solution described as containing 5 mL of formic acid and 90 mL of formalin is accurately identified as a formic acid solution. This is because the presence of formic acid (a simple carboxylic acid) in combination with formalin (which is a solution of formaldehyde in water, typically containing methanol) qualifies the mixture primarily as a formic acid solution.

Formic acid is often used in histopathology for its properties in preserving tissues and assisting in the fixation process. The specific combination of these two substances could be utilized in tissue fixation protocols, where formic acid enhances the penetration of the formaldehyde, potentially providing better morphological preservation and clear histological results.

The other options do not align with the identification of the solution. Nitric acid formaldehyde refers to a different chemical composition and application, whereas aqueous nitric acid and trichloroacetic acid also denote entirely different reactive agents. Therefore, the definitive presence of formic acid paired with formalin distinctly categorizes the mixture as a formic acid solution.