Final answer:
A safety hood helps Morgan inside a tunnel filled with poisonous gas by providing clean, breathable air, since oxygen is vital for human survival, particularly in a contaminated environment.
Step-by-step explanation:
A safety hood, such as a Nomex hood, helps Morgan inside a tunnel filled with poisonous gas primarily by option A) It provides oxygen for Morgan to breathe. Nomex hoods are used in racing and firefighting and are designed to protect users from extreme heat, but in the context of poisonous gas, the purpose of a safety hood or breathing apparatus is to supply the wearer with breathable air in an environment where the air is contaminated. This is because oxygen is crucial for the production of energy in the body, by oxidizing food compounds. Without a clean supply of oxygen, the wearer, in this case Morgan, would not be able to survive in the poisonous atmosphere of the tunnel.
The hood covers the parts of the head not protected by the helmet and face mask, providing additional safety measures against heat. However, the question seems to be focused on the immediate danger presented by the presence of a toxic gas. The most plausible function of the safety hood in this scenario is to ensure a constant provision of clean air, likely through a connected air source or filter that removes the toxic elements of the surrounding atmosphere.