Final answer:
The correct pronoun to fill in the sentence is 'They'.
Step-by-step explanation:
The correct pronoun to fill in the sentence is option A: They. The sentence should read, 'They has eaten my dessert!'. The pronoun 'they' is used as a plural pronoun to refer to more than one person or thing. In this case, it is used to replace the unknown subject who ate the dessert.
Learn more about Using the correct pronouns in a sentence