Answer:
D: Sabía.
Step-by-step explanation:
Juan sabía la verdad. - It is sabía because sabía is the imperfect form of saber in the yo and él/ella/usted forms. The one being used here is the él/ella/usted, because Juan is someone that is not yourself or multiple people, therefore it can only be "him/her". The word usted isn't used either so it cannot be the usted form.
sabia (without the acute accent over the i) means "wise", and sabían is the they/them (ellas/ellos) conjugation form of the same verb.
Also note that the accent influences pronunciation. sabía is pronounced sabía, whereas sabia is pronounced like sabia (If I am remembering correctly, anyway), with the bolded parts indicating the stressed syllable.
Hope this helped!