We ... (not to visit) our granny last Friday.

The sentence describes a past event that did not happen. The simple past tense is used for specific past actions or events. Therefore, 'didn't visit' is the correct form to indicate a past action that was not performed. The other options are incorrect because 'weren't visiting' is past continuous, 'hasn't visited' is present perfect (singular subject), and 'haven't visited' is present perfect (plural subject), none of which fit the context of a specific past time ('last Friday').

Final Answer: didn't visit