Which types of gases are associated with IMO Type 5 or Specification 51?

The correct choice is that propane and butane are associated with IMO Type 5 or Specification 51. These gases are classified as liquefied gases that can be transported under pressure, making them suitable for this type of container specification.

IMO Type 5 tanks, which fall under the International Maritime Organization regulations, are specifically designed to carry certain liquefied gases at various temperatures and pressures. Propane and butane are commonly transported in specialized containers due to their properties — they remain in liquid form when under pressure, allowing for efficient storage and transport.

Other gases mentioned in the other options do not align as closely with the specifications and requirements of type 5 containers. For instance, while helium and oxygen are gases frequently used in various applications, they do not fit the category of liquefied gases that require the specified containment of Type 5. Similarly, hydrogen and nitrogen, though important gases, have different storage requirements, and natural gas and carbon dioxide pertain to different classifications within transportation regulations. Therefore, propane and butane are the correct answer, as they match the requirements for the transport of liquefied gases.