Which of the following is an operational nuclear power station in Britain?

Hinkley Point B is indeed an operational nuclear power station in Britain. Located in Somerset, this station has been a key player in the UK's energy production for many years, contributing to the electricity supply. It has been designated as an essential facility for generating low-carbon energy and has been in operation since the 1970s.

On the other hand, Dungeness B was taken offline for maintenance and is in the process of decommissioning. Sizewell C is still under development and not yet operational, making it premature to consider it a functional power station. Forsmark is located in Sweden and is not relevant to the question regarding British nuclear power stations. Therefore, Hinkley Point B stands as the only correct answer among the choices listed.