Explanation
Correct answer: D.
Choice D is the best answer. “Additionally” logically signals that the painting analysis discussed in this sentence is an additional part of the botany research discussed in the previous sentence. That is, to research which fruits Umbrians grew in the past, the botanist analyzes old paintings in addition to looking at old lists of ingredients.
Why the other choices are wrong
Choice A
Choice A is incorrect because “in sum” illogically signals that the painting analysis discussed in this sentence is a concluding summary of the botany research discussed in the previous sentence. Instead, the painting analysis is an additional part of that research.
Choice B
Choice B is incorrect because “instead” illogically signals that the painting analysis discussed in this sentence is an alternative to the botany research discussed in the previous sentence. Rather, the painting analysis is an additional part of that research.
Choice C
Choice C is incorrect because “thus” illogically signals that the painting analysis discussed in this sentence is a result of the botany research discussed in the previous sentence. Instead, the painting analysis is an additional part of that research.