Choose the correct answer. It isn't allowed to take photographs in the museum.

The phrase "It isn't allowed" indicates a prohibition. Among the given options, "You mustn't take photographs in the museum." and "You can't take photographs in the museum." and "You may not take photographs in the museum." express a strong prohibition. "You mustn't" and "You can't" are the most direct equivalents.

Final Answer: The final answer is $$oxed{You mustn't take photographs in the museum.}$$