The card is chosen at random as the ability resolves. If any player responds to the ability, that player won't yet know what card will be chosen.

Because the last ability doesn't target any card in your graveyard, any card put into your graveyard in response to the ability may be chosen.

Each card in your graveyard must have an equal chance of being chosen. Assigning them each a number and rolling a die is an easy way to do this. In formats that allow you to rearrange your graveyard—including Standard, Modern, and Return to Ravnica block Limited formats, you may also shuffle your graveyard face down and choose a card at random.