The correct word to complete the sentence is A. ibamos.
So the complete sentence would be: "De pequeño, mi familia íbamos a la casa de mis abuelos todos los domingos."
However, note that the subject "mi familia" (which is singular) should take the third-person singular form, so the proper form should actually be B. iba.
Thus, the most appropriate completion considering grammatical agreement is B. iba.
The correct phrase is: "De pequeño, mi familia iba a la casa de mis abuelos todos los domingos."