You are an expert Python programmer.
Write a Python code that defines a function named `solve` which takes one argument:
- a JSON string representing a list of objects (dictionaries).
The code must:
- parse the JSON string,
- return a list of all objects that contain a key "active" with value True,
- raise a ValueError if the string is not valid JSON,
- you may import the built-in 'json' module if needed.
Important formatting rules:
- Output only valid Python function code, nothing else.
- Enclose the code between <python_code> and </python_code> tags.
- Do not add explanations, comments, or test case.

