Headlines and Titles of Works

Q. Do you capitalize the preposition for in headline-capitalization style in this case: “XYZ: what is it good for?” Lowercase or uppercase? Thanks a lot!

A. The last word is capitalized in a Chicago-style headline-capped title, regardless of syntax: “XYZ: What Is It Good For?” Please see CMOS 8.159 (point 1) for this rule.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]