What fraction can be multiplied and equal 4/5

Refer to interactive solution 17.45 to review a method by which this problem can be solved. the fundamental frequencies of two air columns are the same. column a is open at both ends, while column b is open at only one end. the length of column a is 0.504 m. what is the length of column b?

Which of the choices shown could be used to prove that aacp=abcp ?

Me. i can't figure out this question. give an explanation too. .