Based on the excerpt provided, the most likely inference about Mrs. Rachel's character is that Mrs. Rachel distrusts Marilla and Matthew Cuthbert.
Her initial shock and disbelief, followed by her exclamation of "A boy!" and her statement that the world is "turning upside down," suggest that she finds this decision to be unconventional and unexpected. The fact that she considers it "unheard of" further emphasizes her surprise and disbelief.
While the excerpt does not provide enough evidence to definitively conclude that Mrs. Rachel distrusts Marilla and Matthew Cuthbert, her surprise and skepticism do hint at a lack of understanding and perhaps even disapproval of their decision.
"Are you in earnest, Marilla?" she demanded when voice returned to her.
"Yes, of course," said Marilla, as if getting boys from orphan asylums in Nova Scotia were part of the usual spring work on any well-regulated Avonlea farm instead of being an unheard of innovation.
Mrs. Rachel felt that she had received a severe mental jolt. She thought in exclamation points. A boy! Marilla and Matthew Cuthbert of all people adopting a boy! From an orphan asylum! Well, the world was certainly turning upside down! She would be surprised at nothing after this! Nothing!
A.
Mrs. Rachel distrusts Marilla and Matthew Cuthbert.
B.
Mrs. Rachel wants to go to Australia to see kangaroos.
C.
Mrs. Rachel prefers the company of little girls to boys.
D.
Mrs. Rachel is awestruck by the idea of an adoption.