G6A12:What is the primary purpose of a screen grid in a vacuum tube?

Direct answer summary (2023-2027 NCVEC pool)

The official correct answer for G6A12 is To reduce grid-to-plate capacitance.

The key fact is To reduce grid-to-plate capacitance. Match that fact to the exact condition in the question; the other choices describe a different rule, band, component, or operating situation.