I guess you refer to a sheet containing the right answers to the test and not the blank template where you should write your answers. 'Clave' is usually employed as a synonym to 'contraseña' that translates as 'password'. It's not usual to refer to an answer sheet as 'las claves' o 'hoja de claves' but it would be understood. 'Hoja de respuestas' refers exactly to the blank template where the student writes down his answers to the test and not to the printed one with the solutions. This one is usually called 'planilla de respuestas' o just 'planilla' (specially if it is a punched page with holes for the right answers). 'Clave' can also mean 'key' in the meaning of something central and determinant for a given concept. But never as a physical thing that can open a lock. The only way to refer to that concept is 'llave'. Is interesting than both 'clave' and 'llave' come from the same latin word 'clavis'.