You conduct experiments to change the DNA of the flax plant in hopes of breeding a stronger source of linen. You notice that a single change to a gene causes the sequence of protein A to become drastically longer. What could that change be?

Answer: there has been a change of the stop codon.

Explanation:

The stop codon is necessary to stop the synthesis of a protein. When protein A becomes longer than it used to be, it is an indication that the stop codon has been changed hence the increased synthesis of protein A.