Answer:
Use of litmus papers
Step-by-step explanation:
Since it is already known that among the two bottles containing unknown solutions; one contains hydrochloric acid while the other contains sodium hydroxide. The student only needs to perform a simple test to ascertain which of the bottles contains the acid or the base respectively.
A base is known to turn red litmus paper blue and an acid is known to turn blue litmus paper red. Hence by inserting moist red or blue litmus papers into each solution, the student can be able to ascertain which is which and replace the labels appropriately.