Answer:
So diligent is the error in the sentence.
Step-by-step explanation:
Diligent is an adjective while diligently is an adverb, therefore they have a different function in a sentence, the adjective will affect a noun, and an adverb will mostly affect a verb, in the sentence "When you completed your homework so diligent" the word that is being affected is the verb completed so the word to describe how did the person do the action is the adverb diligently.