Final answer:
Usher wanted to preserve Madeline's corpse mainly due to the unusual nature of her illness and their mysterious twin connection, as well as his fear of inquiries from doctors and the end of his family line. The correct answer is A) To study her illness.
Step-by-step explanation:
The reason Usher wanted to preserve Madeline's corpse was primarily due to the unusual character of her malady, and to avoid the inquiries of medical professionals, given the rarity and strangeness of her condition.
Moreover, Usher had an intensely psychological and possibly supernatural connection with Madeline, as they were twins with a deep, empathic bond.
The preservation of her body could also be interpreted as a reflection of Usher's inability to let go and as part of his morbid fear and anticipation of his family line ending with Madeline's death.
This intention to delay her burial reveals his own psychological struggles and perhaps a denial or hope against the finality of death, especially given the uncanny nature of Madeline's illness and its cataleptical symptoms that made her seem eerily lifelike even in death.