Final answer:
The preposition 'por' should be used in the phrase 'cuarenta ciento'
Step-by-step explanation:
The correct preposition to use in this phrase is por. When expressing a percentage, the preposition por is used in Spanish. For example, in the phrase 'cuarenta por ciento' (40%), 'por' indicates multiplication or division. So, in this case, it would be 'cuarenta por ciento'.
Learn more about Prepositions in Spanish