It is easier to determine this if we draw the structural formula of lactic acid as shown in the attached picture. There are three functional groups in lactic acid: carboxyl group, hydroxyl group, and the parent alkane chain. Any of the hydrogens in the alkane chain is the least acidic. Then, it is followed by the H in the hydroxyl group. The most acidic is the H in the carboxyl group.
Explanation:
a) blue litmus paper turns red when u add acid in it .
b) red litmus paper , when an alkali dissolves in water it produce hydroxide ions
which cause it to become a alkaline