The correct option is C a river
Medieval towns tended to grow around areas where people could easily meet, such as crossroads or rivers. Towns needed more water than villages, so a nearby water supply was vital. Rivers would provide the water used for washing and drinking and they were used for the disposal of sewage (if it had not been simply thrown into the streets). Hence, Option A is correct. Farms were a miniscule part of a town. An isthmus is a narrow strip of land that connects two larger landmasses and separates two bodies of water. It contained a number of countries. A cathedral was another miniscule part of both towns and villages. Since none of these other features were central to the formation of medieval towns, these are incorrect.