Final answer:
The missing reason can be filled by using the Midsegment of a Triangle Theorem, which states that the line segment connecting the midpoints of two sides of a triangle is parallel to the third side and half its length.
Step-by-step explanation:
The missing reason can be filled by using the Midsegment of a Triangle Theorem. According to this theorem, the line segment connecting the midpoints of two sides of a triangle is parallel to the third side and half its length. In this case, segment DE connects the midpoints of sides AB and AC of triangle ABC. Therefore, DE is parallel to BC and half its length. Hence, we can use the Midsegment of a Triangle Theorem to fill in the missing reason.