Based on the information provided, the observation that would provide the best evidence to support Fernando's conclusion that some magnets are stronger than others is the difference in the number of steel paper clips that the bar magnet and the horseshoe magnet can pick up.
The reasoning is as follows:
1. Comparing the number of paper clips picked up by different magnets is a direct way to measure and compare their magnetic strength or magnetic field strength.
2. If the bar magnet and the horseshoe magnet pick up a different number of paper clips, it indicates that they have different magnetic strengths. The magnet that can pick up more paper clips has a stronger magnetic field.
3. This direct comparison of the magnetic strength of the two different types of magnets provides the best evidence to support Fernando's conclusion that some magnets are stronger than others.
Other observations, such as the shape or appearance of the magnets, would not directly measure their magnetic strength and would be less conclusive evidence to support the given conclusion.
Therefore, the difference in the number of paper clips picked up by the bar magnet and the horseshoe magnet is the observation that provides the best evidence to support Fernando's conclusion.