Step 1: Understanding the corrosion behavior.
The rate of corrosion of mild steel is highest when exposed to concentrated sulfuric acid. This is due to the aggressive nature of concentrated acids.
Step 2: Explanation of options.
- (B) Sulfuric acid at concentrations above 70% is most corrosive to mild steel.
- (A) Dilute acid has a lower corrosion rate compared to concentrated acid.
- (C) Intermediate concentrations of acid are less corrosive than concentrated acid.
- (D) This option is incorrect as the corrosion rate is not unacceptably high for both dilute and concentrated acid.
Final Answer: \[ \boxed{\text{B) acid at concentrations above 70%}} \]