What did Rahab tie in her window as the sign for Israel to spare her household?

Answer: A scarlet cord

The spies told Rahab to tie the scarlet cord in the window and gather her family inside; after they left, she tied it there.

“unless, when we enter the land, you have tied this scarlet cord in the window through which you let us down, and unless you have brought your father and mother and brothers and all your family into your house.”— Joshua 2:18 (BSB)