Why The first Fresnel zone is the ellipsoid around the line-of-sight path where the bulk of the signal energy travels, and its radius grows with wavelength: r1 = 17.31 * sqrt(D / (4f)) meters, with D in km and f in GHz. Radius varies as 1/sqrt(f), so the highest frequency listed has the smallest zone. At 5.8 GHz the wavelength is only about 5 cm, so the zone is narrower than at any of the lower bands and clears obstacles more easily for a given path.
Watch out 900 MHz is the tempting pick if you reason that a lower frequency 'bends around' obstacles better, but that is diffraction, not zone size; its long wavelength actually makes the Fresnel zone the largest of the group.
Fresnel zone size follows wavelength: highest frequency, smallest zone. Pick the biggest GHz number.