Step 1: List what a good ornament metal needs.
An ornament metal should look attractive, should not react and spoil over years, and should be workable into fine, intricate designs.
Step 2: Check gold against each requirement.
Gold has a bright, shiny yellow lustre, so it satisfies the looks requirement. Gold sits at the very bottom of the reactivity series, so it does not react with air, water or common acids, meaning it will not tarnish over years, satisfying the durability requirement. Gold is also the most malleable and ductile metal known, it can be beaten into thin sheets or drawn into fine wire, satisfying the workability requirement.
Step 3: See that all three properties matter together.
None of these three properties alone would be enough to explain why gold is chosen, jewellers need the shine, the permanence, and the ability to shape it, all at once.
Step 4: Conclude.
Gold is used for ornaments because it is lustrous, unreactive and malleable, all of these. \[ \boxed{\text{All of these}} \]