Frostboil Snarl
Land
As this land enters, you may reveal an Island or Mountain card from your hand. If you don't, this land enters tapped.
: Add 
or 
.
A smoldering storm of chilling fire.
Sam Burley
Rulings
If a land card with an appropriate subtype is entering the battlefield from your hand at the same time as one of these lands, you may reveal the other land to have the "Snarl" enter untapped.
(wotc • 2021-04-16)
The "Snarl" itself doesn't have any land subtypes. You can't reveal one to satisfy the ability of another.
(wotc • 2021-04-16)
If an effect instructs you to put one of these lands onto the battlefield tapped, it will still enter the battlefield tapped even if you reveal a land card from your hand.
(wotc • 2021-04-16)
You may reveal any land card with either or both of the appropriate subtypes. It doesn't have to be a basic land card.
(wotc • 2021-04-16)
Rulings
If a land card with an appropriate subtype is entering the battlefield from your hand at the same time as one of these lands, you may reveal the other land to have the "Snarl" enter untapped.
(wotc • 2021-04-16)
The "Snarl" itself doesn't have any land subtypes. You can't reveal one to satisfy the ability of another.
(wotc • 2021-04-16)
If an effect instructs you to put one of these lands onto the battlefield tapped, it will still enter the battlefield tapped even if you reveal a land card from your hand.
(wotc • 2021-04-16)
You may reveal any land card with either or both of the appropriate subtypes. It doesn't have to be a basic land card.
(wotc • 2021-04-16)