Astral Cornucopia

Astral Cornucopia enters with X charge counters on it.
{T}: Choose a color. Add one mana of that color for each charge counter on Astral Cornucopia.

The Brothers' War Retro Artifacts

Astral Cornucopia enters the battlefield with X charge counters on it.
{T}: Choose a color. Add one mana of that color for each charge counter on Astral Cornucopia.

Commander Anthology Volume II

Commander 2016

Astral Cornucopia enters the battlefield with X charge counters on it.
{T}: Choose a color. Add one mana of that color to your mana pool for each charge counter on Astral Cornucopia.

If you choose 1 for the value of X, Astral Cornucopia will cost {3} to cast and enter the battlefield with one charge counter. If you choose 2 for the value of X, it will cost {6} to cast and enter the battlefield with two charge counters, and so on.

The last ability is a mana ability. It doesn't use the stack and can't be responded to.