Final answer:
Trichloroacetic acid is the acid most frequently used to precipitate protein for confirmatory urinalysis tests.
Step-by-step explanation:
The acid most frequently used to precipitate protein for confirmatory urinalysis tests is d) Trichloroacetic acid.
Trichloroacetic acid is a strong organic acid that is commonly used in laboratory settings to precipitate and isolate proteins from biological samples, including urine.
It is effective in denaturing proteins and promoting their precipitation.