Based on the information provided:
- Jamal has proven that \( MN = \frac{1}{2} AC \) and that \( MN \parallel AC \) by using the Triangle Midsegment Theorem.
- He has also established that \( \triangle QBN \sim \triangle PBC \) by the AAA Similarity Theorem.
Since \( MN \parallel AC \) and \( QN \) is a segment that corresponds to \( AC \) in the similar triangles \( \triangle QBN \) and \( \triangle PBC \), we can use the properties of similar triangles to find the ratio of their corresponding sides.
Because \( MN \) is the midsegment and is equal to half the length of \( AC \), this indicates that the corresponding side \( QN \) would also be half the length of the side it corresponds with in triangle \( PBC \).
Thus, if \( MN \) (which is half of \( AC \)) is parallel to \( AC \), then the length \( QN \) should also be similarly proportional to \( AC \).
From \( \triangle QBN \sim \triangle PBC \), since the similarity ratio is preserved, we conclude: \[ QN = \frac{1}{2} AC \]
So, the correct response is:
QN = \( \frac{1}{2} AC \).