Put in one or ones.
Example: Just look at those T shirts! Which one?

1. Which T shirts? Which ?
2. The T shirts in that shop window over there. Which ?
3. Oh yes. They´re nice. I like the red T shirt. Which ?
4. I do, too. But I think the blue T shirt is nice, too. Which ?
5. Let´s go inside. I´ll try the red T shirt. Which ?