A good strategy is to set up a little diagram, in which you position the different vegetables.
Kale > Spinach Collard Greens > Lettuce
Conclusion: Kale > Lettuce
We can go through each answer choice and see which one does not reinforce the diagram above.
Starting with (A) we can see that if collard greens have more nutritional value than kale, then there is no way we can determine whether kale is healthier than lettuce:
Collard Greens > Lettuce > Kale
Collard Greens > Kale > Lettuce
Either scenario is valid. However, the first scenario goes against the conclusion and is therefore the answer.
Hope that helps
Magoosh Test Prep