Final answer:
The student can use the Transitive Property to prove that 21 and 23 are congruent. Hence the correct answer is option C
Step-by-step explanation:
To prove that 21 and 23 are congruent, the student can use the Transitive Property. This property states that if a=b and b=c, then a=c. In this case, the student can show that 21=23 and 23=21, therefore they can conclude that 21=23 using the Transitive Property.
Hence the correct answer is option C