In free space, how does the gain of two three-element, horizontally polarized Yagi antennas spaced vertically 1/2 wavelength apart typically compare to the gain of a single three-element Yagi?
The key fact is Approximately 3 dB higher. Match that fact to the exact condition in the question; the other choices describe a different rule, band, component, or operating situation.
Memory hook: A half wave dipole is two quarter wave legs.
Wavelength in meters = 300 ÷ frequency in MHz