Here's why:
* Ambiance refers to the atmosphere or feeling of a place.
* The sentence describes the bistro's atmosphere as "youthful," which makes sense in the context of ambiance.
The other sentences don't make sense with the word ambiance:
* "The singer made an off-key ambiance" doesn't make sense. "Off-key" describes a sound, not an atmosphere.
* "I bought you wanted?" is an incomplete sentence and doesn't include the word ambiance.