In general, electrical conductivity is directly proportional to the charged or ionic content of the medium. In other words, the more charged ions or chemicals that are present in a given medium, the higher the electrical conductivity will be. So let's look at each option in turn.
A. Salt is chemically defined as NaCl and is made up of positively charged sodium ions (Na) and negatively charged chloride ions (Cl). It's presence would definitely increase conductivity.
B. Erosion of rocks and minerals produces salts and ionic species that leech into the groundwater, thus increasing conductivity.
C. Photosynthesis is the process whereby plants take sunlight and carbon dioxide to produce oxygen and glucose. Neither oxygen or glucose are ionic or charged chemical species, so photosynthesis would be unlikely to increase conductivity.
D. Fertilizer is most commonly made up of inorganic salts like ammonium nitrate. It would definitely aid in conductivity.
The correct answer is therefore C.