Headlines and Titles of Works

Q. Do you capitalize the preposition for in a title like this one: “XYZ: what is it good for?” Lowercase or uppercase? Thanks a lot!

A. The last word is capitalized in a Chicago-style title, regardless of syntax: “XYZ: What Is It Good For?” Please see CMOS 8.160 (rule 1).

[This answer relies on the 18th edition of CMOS (2024) unless otherwise noted.]