Right answer: conocido
This sentence is written in Preterite pluperfect tense, also called pluperfect or Past Perfect which is used to talk about actions that were completed before some point in the past and in spanish has the following structure:
Subject + had + main verb in past participle + complement
Note in spanish the verbs must be conjugated according the subject and must match in genre and number.
So, the sentence is written with the third person in plural ellos (they), which are the Ramirez, therefore the correct conjugation of the verb conocer (to meet, in this context) in Past Perfect tense is:
habían conocido
And the complete sentence is:
Los señores Ramirez ya se habían conocido antes de conocer a mi mamá
Messrs. Ramirez had met before they met mom
