Strong acids can be distinguished from weak acids by any of the following methods, EXCEPT

Explanation

Strong and weak acids can be distinguished by their pH value, electrical conductivity, and reactivity.

A conductivity measurement can distinguish between strong and weak acids as strong acids have a higher concentration of H+ and ionize completely in water, resulting in a higher concentration of ions and greater electrical conductivity. Weak acids only partially ionize, resulting in fewer ions and lower electrical conductivity. 

Generally, all acids turn blue litmus red.

Strong acids have lower pH values than weak acids. The pH scale measures the concentration of H+ in a solution, and the more H+ present, the closer the pH is to 0. Thus, measurement of pH can be used to distinguish between strong and weak acids.

The measurement of heat of reaction can distinguish between strong and weak acids.Strong acids have a higher enthalpy of neutralization than weak acids. This is because strong acids completely ionize in water, releasing all of their protons (H+ ions), while weak acids have a lower enthalpy of neutralization than strong acids. This is because weak acids only partially ionize, releasing a small fraction of their protons.

Consequently, since the use of litmus paper can't tell us how strong a solution is, option B is the correct answer. 

There is an explanation video available .