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icono de clave de la respuesta
icono con/para la clave de la respuesta
icono clave respuesta (grammatically incorrect - more like part of a telegram but it may be the sort of thing you want, it being shorter and more direct)
Explanation: If you just want to name that icon, i.e. to put a name to it so that the user knows what it means, I would simply say "soluciones", since this is what it says in exercise books in Spanish. If you want to refer to the icon, I would say "icono de soluciones".
I think "soluciones" is better understood than "respuestas". I also like "claves", which is also correct but not as much used as "soluciones".
I've done some translation work in e-learning and this is what I use