The correct answer is
Multiply 5.3 by the speed of light in kilometers per hour
In fact, the light moves by uniform motion, so we can use the relationship
[tex]v= \frac{S}{t} [/tex]
where S is the distance covered by the light, t the time it takes, and v the speed of light.
So, if we want to find the distance between Sun and Pluto,
[tex]S=v t[/tex]
which means that we should multiply the time (5.3 hours) by the speed of light in km/h.