"The dog is fat."
What about 뚱뚱해요? Shouldn't this be accepted as the informal correct answer? It told me it was wrong.
뚱뚱하다 is the verb infinitive and isn't usually used. 뚱뚱합니다 is the formal conjugation.
The meaning doesnt change it's just different conjugation 합니다 is conjugated into an honorific form 하다 is both the declarative form and the infinitive form (aka not conjugated)