Looking at the answers of this question heavily borrowed from the Radio-Nav syllabus:

'Diffraction of ground waves and ionospheric attenuation' --> INCORRECT. VHF is not a ground wave.

'Ionospheric attenuation and the tropospheric conditions' --> INCORRECT. The ionosphere does not usually reflect VHF radio and thus transmissions are restricted to a local area.

'Tropospheric conditions and attenuation as a result of atmospheric absorption' --> CORRECT.

VHF propagates as quasi-optical, direct wave. The VHF waves are space waves (not sky waves). Unlike High Frequencies (HF). During the radio wave travel, its energy is taken up by the ground or atmosphere (hail, ionosphere, rain, etc). This way the electromagnetic energy is transformed into other forms of energy. Consequently, the signal becomes weaker as energy is absorbed by the Earth and the atmosphere.

'Attenuation as a result of atmospheric absorption and diffraction of ground waves' --> INCORRECT. VHF is not a ground wave.