New Orleans was vulnerable to flooding when Hurricane Katrina struck primarily because much of the city is located below sea level. This geographical characteristic, combined with the city's surrounding levee systems that were inadequate or breached, contributed significantly to the flooding that occurred during and after the hurricane.
Additionally, while the other options may point to various factors related to the city's situation and response, the key vulnerability stems from the fact that much of New Orleans is below sea level, making it particularly susceptible to flooding from storm surges and heavy rainfall.