Answer: A. is deactivated in a weakly acidic solution
Step-by-step explanation: pH is the measure of acidity or alkalinity. 
For acidic solutions the pH ranges from 1 to 6.9, for neutral solutions the pH is 7 and for basic solutions pH ranges from 7.1 to 14.
![pH=log(1)/([H^+])](https://img.qammunity.org/2018/formulas/biology/high-school/46b4vimjta5aecla9zc0qcegqox29d4k33.png)
As pH and 
 are inversely related, more is the concentration of 
 ions, lesser is the pH and vice versa. Thus weakly acidic medium with low concentration of 
 ions have high pH and strongly acidic medium with high concentration of 
 ions have low pH.
As can be seen from the graph, The enzyme works optimally in strongly acidic conditions and is deactivated as the acidity decreases. The enzyme works optimally in strongly basic medium.