According to Ezra 1:1, Cyrus's proclamation fulfilled the word spoken through which prophet?

Answer: Jeremiah

Ezra opens by saying Cyrus's decree fulfilled the word of the LORD spoken through Jeremiah.

“In the first year of Cyrus king of Persia, to fulfill the word of the LORD spoken through Jeremiah, the LORD stirred the spirit of Cyrus king of Persia to send a proclamation throughout his kingdom and to put it in writing as follows:”— Ezra 1:1 (BSB)