Prolog checks if ripon$\_$star is a roan by evaluating the rule roan$($X$)$ :$-$ not paint$($X$),$ not clay$\_$bank$($X$).$ For roan$($ripon$\_$star$)$ to be true, ripon$\_$star must not be a paint and must not be a clay$\_$bank.

Prolog does not have any facts stating that ripon$\_$star is a paint or a clay$\_$bank.

Due to the closed$-$world assumption, Prolog assumes that anything not explicitly stated as true is false.

Therefore, since ripon$\_$star is neither explicitly a paint nor a clay$\_$bank, Prolog concludes that roan$($ripon$\_$star$)$ is true.