What weather element caused the reduction in visibility according to the PREP: OKC UA/OV OKC180010/TM 1516/FL120/TP BE20/SK BKN035 TOP075/OVC095-TOPUNKN/WX FV01SM SN/TA M04/TB MOD 050-070/RM TCU W DURC?

The correct answer is based on the weather conditions described in the notation. The term "WX FV01SM SN" indicates that there is falling snow, which directly points to snow as the weather element impacting visibility. Snow can significantly reduce visibility when it is falling, particularly if it's moderate or heavy, as it can obstruct the view and create a whiteout effect in certain conditions.

In this scenario, the presence of snow is made clear by the abbreviation "SN", which denotes snowfall. This helps establish that the reduction in visibility is directly related to the weather phenomenon of snow. Other potential weather elements that might reduce visibility, such as fog, rain, or haze, are not indicated in the provided report. Therefore, snow stands out as the primary factor causing the reduction in visibility.