The location that should have the highest temperature after thirty minutes is B: the cup in a sunny spot on the grass.
This is because direct sunlight provides the most heat and energy to the water, raising its temperature compared to other locations that lack direct sunlight. In contrast, the other locations either have indirect sunlight (like the cup in the shade of a tree), no light (the dark corner of the classroom), or are indoors in a potentially cooler environment (the desk in the classroom).