Як is more like "which," except that in Ukrainian the meaning is completely different to ask literally, "What is your favorite season?" That would be like asking what does "your favorite season" mean. The proper way is
Which is your favorite season? Hence,
Яка твоя улюблена пора року?
The translation should be 'Which is your favourite season?'. However, in English common usage in conversation is 'What is your favourite season?' In formal writing the 'what' would be corrected to 'which', as it is asking which one out of a number of possibilities is your favourite. Ideally, both should be accepted, as both are in use in English, but it depends on the moderators' interpretation of how to best represent the question in English.