Final answer:
The correct form of the adjective to use is allemands.
Step-by-step explanation:
The correct form of the adjective to use in this sentence is allemands.
Since films is plural, we need to use the plural form of the adjective. In French, adjectives agree in gender and number with the noun they modify. Since films is masculine plural, we use the masculine plural form of the adjective allemands.
An example sentence using the correct form would be: Mon cousin préfère les films allemands.
Learn more about French adjectives