The leader whose's picture is in the top right will pick an option. The manual says that they will usually pick "the safest option" (there is a default option defined for decisions) but may pick an option that corresponds more to their profile preferences instead depending on their Relation score and Authority personality trait. You'll be notified of the choice with a 'Decision made in your name' message in the letterbox the next turn.
Not affiliated with Slitherine. They added it to my name when they merged the Slitherine and Matrix account systems.
There is two options:
1) If there is a default decision defined. That one is picked
2) The Secretary picks, based on matching profiles. And as a fallback, entirely random.