Step-by-step explanation:
The boy with the help of litmus paper can easily decide it as follows:
(i) The drink that would turn a red litmus to blue would surely be basic in nature.
(ii) The drink that turns a blue litmus to red is be acidic in nature.
(iii) The drink that would not be affected by both red and blue litmus would is neutral in nature.
The test can be done with help of phenolphthalein or the methyl orange these reagents also show different color in different medium.