Formal:
* Qu'est-ce que c'est ? (pronounced: kɛst sə kɛ) - This is the most common and formal way to ask.
Informal:
* C'est quoi ? (pronounced: sɛ kwa) - This is a more casual way to ask, similar to "What's this?" in English.
* C'est quoi ça ? (pronounced: sɛ kwa sa) - This is another informal way to ask, emphasizing the object you're pointing at.
With a question word:
* Quel est ceci ? (pronounced: kɛl ɛ sɪsi) - This translates literally to "What is this?" and is more formal than the other options.
* Qu'est-ce que c'est que ça ? (pronounced: kɛst sə kɛ sa) - This translates to "What is that?" and is more informal than the previous option.
The best choice will depend on your level of formality and the situation.