Some question words (interrogative pronouns) in Russian decline.
Here is how the question words кто (who), что (what), какой (what kind of), чей (whose) change their endings to express a particular cas: Nominative, Genitive, Dative, Accusative, Instrumental, or Prepositional).
Practice
Exercise 1.
Exercise 2. Choose the correct question word.
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Check answer 3
Sorry. What answer do you mean? I have just checked, and all the answers in the tests seem correct.