The acidity of a substance can be determined by its pH value, with lower pH values indicating higher acidity.
Here are the pH values for the options given:
A) Blood with a pH of 7.4 - slightly alkaline B) Rain water with a pH of 5.5 - acidic C) Bleach with a pH of 13.0 - very alkaline D) Sea water with a pH of 8.0 - alkaline
Among these substances, rain water with a pH of 5.5 is the most acidic. Therefore, the answer is:
B) rain water with a pH of 5.5