Final answer:
Dangerous goods carriers are not prohibited from transporting non-dangerous goods or from transporting goods by air or sea, provided they comply with safety regulations. The prohibition mainly involves transporting certain goods, like intoxicating liquors, in violation of local laws.
Step-by-step explanation:
Those involved in the transportation of dangerous goods are prohibited from doing several things, including violating specific laws regarding the distribution of such goods. For instance, carriers of dangerous goods must adhere to strict regulations that may include the prohibition of certain types of goods, such as intoxicating liquors, being transported into certain territories if it violates local laws. However, the question as to what dangerous goods carriers are prohibited from, the options provided are not completely accurate in the context of general prohibition. Dangerous goods carriers are primarily tasked with the transportation of dangerous goods, thus option 1 is incorrect. They are not prohibited from transporting non-dangerous goods, so option 2 is also incorrect. Transporting goods by air or by sea, options 3 and 4 respectively, are permitted as long as the carriers comply with safety and legal standards for those modes of transport. The global transportation network includes shipping by air and sea, even for dangerous goods, when all safety regulations and requirements are met.