Can Zhonya's be activated/used when an opponent targets a unit that is at base (e.g., with Singularity)?

Ruling: Zhonya's is not an activated ability—it's a mandatory replacement effect that triggers whenever one of your units would die, regardless of where that unit is located (including at base). You can also play Zhonya's from hidden as a reaction to any action, and it will wait at base to replace the next death.

Sequence:

• When a unit would die (anywhere, including at base), Zhonya's replacement effect automatically occurs
• The unit is recalled to base instead of dying
• If the unit is already at base, the "recall" part does nothing, but the replacement effect still prevents the death

Nuances:

• Gear played from hidden (like Zhonya's) are only restricted to specific battlefields when choosing targets for "when you play this" abilities
• Zhonya's played from hidden simply goes to base and waits for the next death to occur anywhere