Answer: The right answer is "fui."
Explanation: Just to elaborate a little on the answer, it can be added that in this exercise they are asking you to complete the sentence with the correct form of the verb "ir" (to go). Since the subject in the sentence is the first person singular (yo/I), and the action was completed in the past (the subject went to visit her cousins in Antigua Guatemala), it is best to use the so-called pretérito perfecto simple, "fui," instead of the pretérito imperfecto, "iba" (which could be translated as "I was going"). Fuiste is the form for the second person singular, and íbamos is the form for the first person plural.
