Both the comma after the first word, and the verb in imperative let us know that someone is being addressed directly. In latin, the vocative case is used. For men's names that end in -ius, -i is the marker for the vocative, meaning Juli is the correct answer.

From the singular imperative veni and the exclamation mark, we know that this is a command, and that it is a singular subject; therefore, the subject is being directly addressed, so we must use the vocative case. The correct vocative for nouns ending in -ius is a simple -i, which is not to be confused with nouns that end in -us which take -e as the vocative ending.