Which is a scientific question?
can cats learn to sit down on command?
why do black cats bring bad luck?
are spotted cats cuter than solid-colored cats?
why are persian cats so funny?

"Can cats learn to sit down on command?" Is the scientific question.

The scientific question is "Can cats learn to sit down on command?"

The question "Why do black cats bring bad luck?" is not scientific because luck is an immeasurable quality.

The question "Are spotted cats cute that solid-colored cats?" is not scientific because cuteness/beauty is a highly biased trait, and differs from person to person.

The question "Why are Persian cats so funny?" is also not a scientific question because how funny someone/something is to someone is highly biased and differs from person to person.

The question "Can cats learn to sit down on command?," however, is a scientific question because you can definitively note when the cat responds to a command, and isn't biased.