The correct answer is the first option.
In this context, "strong" is not talking about any physical or mental strength, but instead the player's capacity on the field. We can find this out by re-writing the sentence: The new second baseman is bad at bat, but he's a good fielder. With these adjectives interchanged, it's easier to see that "a good fielder" is talking about the player's abilities, and not his physical or mental strength.
