Issues:
* Word Order: "Longest zipline" should come before "in Asia."
* Article: You need an article before "longest zipline" (the or a).
Corrected Sentences:
* Is the longest zipline in Asia a zipline?** (This assumes you're asking if it's *any* zipline)
* Is the longest zipline in Asia a zipline you've been on?** (This implies you're asking about a personal experience)
* Is the longest zipline in Asia really a zipline?** (This implies you're questioning the accuracy of the claim)
It's also helpful to clarify what you're asking about. For example:
* Is the zipline called "X" the longest in Asia?**
By adding context and using correct word order and articles, your sentence will be grammatically correct and clear.