The most acidic body fluids are A B C D body fluids, saliva, blood, bile and gastric juice. The pH value ranges from 6.6 to 7.1.7.

Analysis: When the PH value of the solution is equal to 7, it is neutral. When the PH value of the solution is greater than 7, it is alkaline. When the PH value of the solution is less than 7, it is acidic.

When the PH value of the solution is less than 7, the acidity increases with the decrease of pH value.

Solution: A, the PH value of saliva is 6.6-7. 1, which is neutral, weakly alkaline or weakly acidic, and the choice is wrong;

B, blood PH 7.35-7.45, weakly alkaline, wrong choice;

C, the PH value of bile is 6.8-7.4, which is neutral, weakly alkaline or weakly acidic, and the choice is wrong;

D. The PH value of gastric juice is 0.8- 1.5, which is very acidic and the choice is correct.

So choose D.

Comments: To solve this problem, we should fully understand the relationship between the acidity and alkalinity of the solution and the PH value of the solution. Only in this way can we make a correct judgment on related issues.