"Which ball is yours?"

Translation: Kiu pilko estas la via?

[TheKinglyWe]

kiu pilk la vias?(can i treat via like an old adjective? this might already of been covered but ive decided to stop reviewing abandon timidity and boldy march forward...)

[vikungen]

Ne, vi nur povas skribi: Kiu pilko estas la via? aŭ Kiu pilko estas via?

[Brucolac]

Kial?

[LouisSepdekdu]

You can't put a verb ending on every adjective and use it like it means "to be + adjective". Doing that will make you write "Mi rapidas" for "I'm fast" when it actually means "I'm hurrying".