Answer:
preterite tense: Conocí al alcalde de la ciudad.
Step-by-step explanation:
In this case the preterite tense is used because meeting the mayor of the city was a one-time action. The sentence above is the direct translation of I met the mayor of the city. The verb conocer is conjugated in the first person preterite tense to reflect this event.
The imperfect form of conocer is used to describe things that someone already knew how to do or was already familiar with at some time in the past.