The correct answer is D. She
Step-by-step explanation:
In grammar, an antecedent is a word, commonly a noun, that can be replaced with a pronoun. In the case of the sentence presented "Sheila wants to study castles so _____ can understand stone construction techniques" the antecedent is "Sheila", this implies the word missing should be "she" as this replaces "Sheila" accurately, considering the pronoun "she" is used for feminine singular nous including proper names of women. Tus, the pronoun that agrees with the antecedent in the sentence is She.