Originally, the ratio of ginger ale to raspberry juice is 2 parts ginger ale to 3 parts raspberry juice. If Matteo adds one part of raspberry juice, the new amounts are:
- Ginger ale: 2 parts
- Raspberry juice: 3 parts + 1 part = 4 parts
So the new ratio is:
2 parts ginger ale to 4 parts raspberry juice.
Thus, the correct answer is: 2 parts ginger ale to 4 parts raspberry juice.