To understand the meaning, we need more context. For example, here are some possible complete phrases and their meanings:
* "Nail or a hammer?" - This asks which tool you need for a task.
* "Nail or a screw?" - This asks which fastener is appropriate.
* "Nail or a pin?" - This asks which object is being referred to.
* "Nail or a staple?" - This asks which fastening method is preferred.
Without more information, it's impossible to determine the meaning of "as" in "nail or a."