Final answer:
The gas that reduces oxygen concentration in blood to dangerously low levels without being toxic itself is nitrogen, due to its ability to displace oxygen in air and lead to hypoxia.
Step-by-step explanation:
A gas that reduces oxygen concentration in blood to dangerously low levels without having toxic or anesthetic properties and without acting upon the heart or hemoglobin is nitrogen (d). Nitrogen is a large component of the air we breathe (around 78%) and is not toxic. However, in high concentrations, it can displace oxygen in the air, leading to a state called hypoxia, where the body is deprived of adequate oxygen supply at the tissue level.
Helium, being a noble gas, is principally non-reactive (chemically inert) and non-flammable. It is not narcotic and does not chemically act on the heart or hemoglobin. However, it is not specifically known to reduce the oxygen concentration in the blood through displacement, unlike nitrogen.
Therefore, breathing air with a significantly increased proportion of nitrogen can lead to asphyxiation due to oxygen depletion, even though nitrogen itself does not have toxic properties. It is for this reason that helium-oxygen mixtures are used for activities like deep-sea diving instead of nitrogen-oxygen mixtures to avoid nitrogen narcosis and maintain appropriate oxygen levels in the blood.