"Did she wait for them at the airport?"
Translation:Počkala na ně na letišti?
In the future, would you please indicate whether the "not" version was rejected?
This exercise is configured to accept both "počkala" and "čekala".
"Počkala" is perfective, so it views the waiting as a completed event. Much of the same feeling is present in the English simple past, "did...wait". Did she get the waiting done?
If you want to know whether at some point of interest to you she was in the process of waiting for them at the airport, "was...waiting" is much more likely or even the predominant way of asking. In Czech, events viewed as being in progress at some past point are usually expressed with the imperfective verb, so "čekala".
The complicating factor is that the waiting got completed once they arrived, and up to that point she was automatically in the process of waiting. For the purposes of Czech aspect, it is not hard to view that single past waiting event both as completed and as ongoing, since we do not have interference from context. The tie-breaker is the feeling of the English simple past tense as more likely to be associated with events viewed as completed, hence "počkala".
In the interest of disclosure, the Czech sentence is primary. This sentence was intended to show "počkala". The real question would have been whether to use "did...wait" or "was...waiting" in the main English translation. No wiggle room there; simple past it is.
These parentheses / / are used to show pronunciation, NOT spelling. See them used, for example, at https://dictionary.cambridge.org/dictionary/english/yellow
Similar are [ ] which are used to mark IPA pronunciation. See also https://linguistics.stackexchange.com/questions/301/when-should-one-use-slashes-or-square-brackets-when-transcribing-in-ipa