To determine who is younger among A and C, let's analyze the given information:
- A is taller than B but shorter than D.
- The tallest girl is the youngest, and the oldest girl is the shortest.
- C is neither the oldest nor the youngest.
From the given information, we can conclude that:
- The tallest girl is the youngest. Therefore, D is the youngest.
- The oldest girl is the shortest. Therefore, A is not the oldest.
- C is neither the oldest nor the youngest.
Based on the given information, we cannot determine who is younger between A and C. Therefore, the correct answer is D - "can't be determined."