Learn Polish: Oni są zmęczeni w trakcie medytacji. - They are tired during meditation.

The phrase w trakcie (meaning 'during' or 'in the course of') always requires the following noun to be in the Genitive case. For the feminine noun medytacja, the Genitive singular is medytacji.

Could I use podczas instead of w trakcie in this sentence?

Yes! Oni są zmęczeni podczas medytacji would mean exactly the same thing. Both podczas and w trakcie mean 'during' and both require the Genitive case. W trakcie can sometimes feel a bit more like 'in the very middle of' an action, but in most everyday contexts they are completely interchangeable.

Why does the adjective take the form zmęczeni instead of zmęczone?

Because the sentence uses the pronoun oni (they), which indicates a masculine personal group (either a group of men, or a mixed-gender group). This requires the masculine personal plural ending for adjectives. If the group consisted entirely of women, children, or animals, you would use the pronoun one and the non-masculine plural adjective zmęczone.

I learned that the verb być (to be) takes the Instrumental case. Why isn't the adjective here in the Instrumental?