To determine the molecular mass of a substance that occupies a volume of 5.6 liters when vaporized at Normal Temperature and Pressure (NTP), it is essential to clarify that NTP refers to conditions of 20°C and 1 atm. The discussion highlights the importance of understanding standard conditions, contrasting NTP with Standard Temperature and Pressure (STP) and Standard Ambient Temperature and Pressure (SATP). The molecular mass can be calculated using the ideal gas law, where 1 mole of gas occupies 22.4 liters at STP, but under NTP conditions, the volume occupied by 1 mole is approximately 24.45 liters. Given the volume of 5.6 liters, the molecular mass can be derived from the relationship between the volume, temperature, and pressure, leading to the conclusion that the molecular mass of the substance is likely to be one of the provided options: M, 2M, 3M, or 4M.