ANS:C - Methane gas

A safety lamp causing flames when inserted into the upper portion of a manhole indicates the presence of methane gas. Methane is a flammable gas commonly found in sewage systems, particularly in anaerobic conditions where organic matter is decomposing. When methane gas is present in sufficient concentration and encounters a flame or spark, it can ignite and cause a fire or explosion hazard. Safety lamps are designed to detect the presence of flammable gases by causing the flame inside the lamp to increase in size or intensity when exposed to such gases. So, the correct option is: "Methane gas."