Does Stifle counter a landfall trigger if cast in response to a land entering the battlefield?

Stifle counters a triggered ability on the stack — it does not prevent a land from entering the battlefield, and it cannot be cast 'in response to' a land entering, because a land entering is not itself a spell or ability that goes on the stack.

Here is the sequence: a land enters the battlefield (this is an event, not a stack object), then any landfall triggered abilities trigger and are placed on the stack (CR 603.2). Only at that point does a triggered ability exist as a target for Stifle. Players then receive priority and may respond (CR 116.5).

So Stifle can counter a landfall trigger — but only after the trigger is already on the stack, meaning the land has already entered. You cannot use Stifle to stop the land from entering, nor can you cast it 'before' the trigger exists.

Concrete example: You play a basic Forest, causing Lotus Cobra's landfall ability to trigger and go on the stack. Your opponent casts Stifle targeting that triggered ability. The trigger is countered and you get no mana — but the Forest is already on the battlefield and stays there.

Relevant rules: CR 603.2 (triggered abilities trigger and go on the stack after the event occurs), CR 603.3 (controller gets priority after trigger is placed), and CR 509.2 / the Stifle card text (counters target triggered or activated ability).