Can you respond to a fetchland activation with a spell?

When you activate a fetchland (like Polluted Delta), the ability goes on the stack as an activated ability. Both players receive priority before it resolves, meaning anyone can cast spells or activate other abilities in response (CR 116.2, 602.2).

For example, your opponent could respond to your fetchland activation with a Stifle to counter the ability, or you could cast an instant in response before you shuffle your library and gain the land.

Once the fetch ability resolves, you sacrifice the land and search — that part happens as part of resolution and cannot be responded to mid-resolution (CR 608.2).

Note: the sacrifice of the fetchland is a cost paid when you activate the ability (before it even hits the stack), so the land is already gone by the time anyone can respond (CR 602.2a).