Final answer:
The bullock cart is slowed down by rain and mud, as these conditions make traditional travel on roads more difficult by causing wheels to sink and get stuck.
Step-by-step explanation:
The direct answer to the question of what slows the bullock cart down on its way to the city is: c) Rain and mud.
An explanation for this answer is that the provided texts describe various difficult conditions for travel, including bad weather and poor road conditions. In the passage referencing the bullock cart, imagery is used to convey a scene of dismal weather that makes the roads slick and muddy. This presents a more challenging environment for the traditional mode of transportation, like a bullock cart, where the combination of rain and mud could likely result in decreased mobility and slower progress due to the wheels sinking and getting stuck.
The references to the other vehicles and animals in the passage are there to set the scene but do not directly impact the speed of the bullock cart in the context given.