Final answer:
The correct form of the verb ser is 'es' to complete the sentence: 'El sándwich de jamón y queso es muy sabroso'.
Step-by-step explanation:
The correct form of the verb ser to complete the sentence is es. In this case, the subject 'El sándwich de jamón y queso' is singular, so we use the singular form of the verb ser which is es. The sentence translates to 'The ham and cheese sandwich is very tasty'.
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