To critique Opal's reasoning, let's analyze the process and the statements made in the proof.
Opal correctly begins with the fact that
The sentence to fill in the blank should emphasize the relationship that arises from the similarity of the triangles. The appropriate reasoning should reflect the relationship between the corresponding sides of similar triangles and incorporate the specific ratio they are dealing with.
Among the available options:
The first option,
With that reasoning in mind, the complete proof would read:
PROOF: Because corresponding sides of similar triangles are proportional,
Thus, the answer that should fill in the blank is