Final answer:
The issue with only having stairs and a hallway door for ventilation in slum tenements was that it resulted in stale and hot air, further worsening the unlivable conditions and contributing to disease.
Step-by-step explanation:
According to How the Other Half Lives, the problem with only having the stairs and the hallway door for fresh air in buildings in the slums was that the air was stale and hot, which is reflected in option a. The poor ventilation left the residents with little access to clean air, exacerbating the already harsh living conditions. The tenements lacked adequate natural ventilation, and with many apartments not having windows or any form of mechanical ventilation, indoor air could not be refreshed, leading to an increased risk of diseases spreading among the inhabitants.