Step 1: Understanding Corrosion Behavior.
Mild steel corrodes most rapidly when exposed to concentrated sulfuric acid due to the acid's aggressive nature.
Step 2: Explanation of Options.
- (B) Sulfuric acid at concentrations exceeding 70% is highly corrosive to mild steel. - (A) Dilute acid exhibits a slower corrosion rate than concentrated acid. - (C) Acids at intermediate concentrations are less corrosive than concentrated acid. - (D) This statement is incorrect because the corrosion rate is not unacceptably high for both dilute and concentrated acid.
Final Answer: \[\boxed{\text{B) acid at concentrations above 70%}}\]