Final answer:
The subject in the sentence is 'Darwin Watterson'. A comma should be added after 'tank' to separate the items in the list.
Step-by-step explanation:
The subject in the given sentence is 'Darwin Watterson'. To make the sentence grammatically correct, we need to add a comma after 'tank' to separate the items in the list: 'a fish tank, blue Jell-O, and Goldfish crackers for his party.'
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