G8C15:What does an FT8 signal report of +3 mean?

Direct answer summary (2023-2027 NCVEC pool)

The official correct answer for G8C15 is The signal-to-noise ratio is equivalent to +3dB in a 2.5 kHz bandwidth.

The key fact is The signal-to-noise ratio is equivalent to +3dB in a 2.5 kHz bandwidth. Match that fact to the exact condition in the question; the other choices describe a different rule, band, component, or operating situation.