Can a land be sacrificed to pay a cost in response to a spell that would destroy it?

When an opponent casts a spell that would destroy your land (such as Stone Rain), that spell goes on the stack and both players receive priority before it resolves. If you have an activated ability or other effect that requires sacrificing a land as a cost, you may activate it in response, sacrificing the land immediately as part of paying that cost.

Costs are paid at the time of activation or casting (CR 116.2, CR 601.2h). Sacrificing a permanent is a valid way to pay a cost (CR 118.12), and it happens instantly — the land leaves the battlefield before the destroying spell ever gets a chance to resolve.

When the destroying spell later resolves, it looks for its target. Because the land is already gone, the target is no longer on the battlefield. Under CR 608.2b, if the target is illegal when the spell tries to resolve, the spell is countered for having no legal targets, and its effect does not happen.

Concrete example: Your opponent casts Stone Rain targeting your Forest. In response, you activate Zuran Orb, sacrificing that Forest to gain 2 life. The Forest is in the graveyard before Stone Rain resolves. When Stone Rain resolves, its target is gone — it is countered on resolution and you keep the 2 life.

Note: This only works if you actually have a legal way to sacrifice the land (an ability or spell with that cost) during your priority window. You cannot sacrifice a permanent arbitrarily without a cost or effect that instructs you to (CR 701.17a).