Since "Ki-Vi" is the most common noun class, I would think that you would use that class. Kenyan Kisahili may differ a little because we use a "neutral class" when the subject is unknown. The class is defined as "I-Zi", so "kinaenda" would become "inaenda" (singular) or "zinaenda" (plural).
The lessons have taught us about pronouns, until now. But here the subject of the sentence is not a pronoun, but a noun (omitted from the sentence). Then, we have to use the subject affix for the corresponding noun class. Here, the subject is from the KI class (KI/VI singular), an the affix for the verb is ki. It would be easier to have all that explained in the lesson, I hope it will be in the final version...