Final answer:
The grammatically correct sentence is 'Yo estoy serio.'
Step-by-step explanation:
The grammatically correct sentence is:
Yo estoy serio.
In Spanish, verb conjugations match the subject. In this case, the subject 'yo' (meaning 'I') requires the verb 'estoy' (meaning 'am') to agree with it. Since 'yo' is singular, we use the singular form of 'estar', which is 'estoy'.
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