Complex sentence: The Brandalism movement continued and in 2015 the most widely known act of Brandism occurred during the time of the 2015 United Nations Climate Change Conference (COP21) in which activists replaced bus-stop advertisements with their own artwork.
Split into simpler sentences: The Brandalism movement continued and in 2015 a more widely known act of Brandism occured. The most current 2015 United Nations Climate Change Conference (COP21) in which activists replaced bus-stop advertisements with their own artwork.