To find out which planet would have a diameter of approximately 12 cm in the scale model where 1 cm is equal to 10,000 km, we can first convert the diameter of the planet from centimeters back to kilometers.
Given that 12 cm corresponds to:
\[ 12 \text{ cm} \times 10,000 \text{ km/cm} = 120,000 \text{ km} \]
Now we can compare this value to the actual diameters of the planets in our solar system. The diameter of 120,000 km is approximately the diameter of the planet Jupiter, which is about 139,820 km in reality.
Thus, the planet with a diameter of approximately 12 cm in the scale model would be Jupiter.