In the English language, the normal order of words in a sentence is: subject + verb + object. The subject is the performer of the action (verb) and the object is the receiver of that action.
In the given sentence, the object "Grant Gustin" is placed before the verb "saw", whereas it should be placed after. Hence the sentence requires change and option A is incorrect.
Option D is correct as the sentence follows the normal word order subject (I) + verb (saw) + object (Grant Gustin).
Option B is incorrect because "I" can't be the object; it's a subject pronoun.
Option C is incorrect because the adverb of place ("Comic con festival" without "at") has been placed as the subject and vice versa. The sentence hence, makes no sense.