Correct choice is (b) Colorimetric
Explanation: A colorimetric method in its simplest form uses only the human eye as a measuring instrument. This involves the comparison by visual means of the colour of an unknown solution, with the colour produced by a single standard or a series of standards. The comparison is made by obtaining a match between the colour of the unknown and that of a particular standard by comparison with a series of standards prepared in a similar manner, as the unknown.
