If the heir to the British throne is female she cannot be the Prince of Wales and the title Princess of Wales is kept for the wife of the Prince of Wales
Actually, changes went into effect regarding the succession in 2015. The requirement was changed from male primogeniture to absolute primogeniture. This means that the first-born, rather male or female, will be the heir apparent. So there can be a Princess of Wales in her own right, though this has yet to happen. "Princess of Wales" should be an alternate answer to the question.
I assume you too thought the question was looking for an official title for any heir of the British throne rather than the title of the next heir. But apparently not. When (if) the Prince of Wales becomes king then the Duke of Cambridge is the next heir.
Mohandas is the most famous Gandhi. Also the hint makes it pretty clear which Gandhi we are talking about. And if that is your objection, why not all these other Gandhis?
https://en.wikipedia.org/wiki/20th_century