2020-01-24 : An ability that triggers when a creature becomes the target of a spell resolves before the spell that caused it to trigger. It resolves even if that spell is countered.

2020-01-24 : If a spell targets Tectonic Giant more than once, its ability triggers only once.

2020-01-24 : If you don't play the exiled card that you chose, it remains in exile with the card you didn't choose.

2020-01-24 : Players can cast spells and activate abilities after the triggered ability resolves but before the spell that caused it to trigger does.

2020-01-24 : Playing an exiled card causes it to leave exile. You can't play it multiple times.

2020-01-24 : Tectonic Giant doesn't change when you can play the exiled card. For example, if you exile a sorcery card, you can cast it only during your main phase when the stack is empty. If you exile a land card, you can play it only during your main phase and only if you have an available land play remaining.

2020-01-24 : You choose which of the exiled cards you'll be able to play while the ability is resolving. You can't wait to pick one later. The other card remains in exile.