"Who drinks sparkling water?"
Translation:Хто п'є воду з газом?
«Вода з газом» literally means 'water with gas'. Ukrainian name refers to the gas CO₂ contained in the sparkling water.
If you drop «з», you'd end up with a bare instrumental case. It's sometimes translated 'with', but it's a different 'with': it's a 'with' used for instruments. «Хто пʼє воду газом?» could mean 'Who drinks water with [=using] gas?'.
In English, you use 'with' both in 'Who eats pasta with [=using] a fork?' and in 'Who eats pasta with [=and] cheese?'. But in Ukrainian, those meanings are kept separate: 'with' in the first meaning is translated with a bare instrumental-case form (виделкою 'with[=using] a fork'), and 'with' in the second meaning is translated using «з» + instrumental-case form (з виделкою 'with[=and] a fork', з си́ром 'with[=and] cheese').
Bere instrumental case has a number other meanings, but none makes sense here. «Хто пʼє воду газом?» could also be translated 'Who drinks water like gas?', 'Who drinks water as gas?', 'Who drinks water by gas'...