Answer:
"Oh, I guess they're not very dangerous things the ladies have picked out."
Explanation:
Susan Glaspell's short play "Trifles" revolves around the murder of Mr. Wright where his wife has been accused of doing the deed and put on trial. The focus of the scene is set in the scene of the crime where the attorney, the sheriff, and Hale were going through the stuff along with their wives.
From the scene given in the question, it is evident that the men did not really think the women are capable of making discoveries, let alone, a crucial one. One evidence is that the lawyer did not think the things that Mrs. Peters has picked to take with her are important or even related to the case. He merely asserts it as "they're not very dangerous things the ladies have picked out", little realizing that it was the women who discovered the reason and possibly uncovered the whole situation relating to the murder.
Thus, the line of dialogue that suggests that the attorney believes women are incapable of plotting is "Oh, I guess they're not very dangerous things the ladies have picked out."