The correct answer is option 'C': The friends decided not to take an oil stove with them because it leaked oil and spoiled the atmosphere. This decision was made due to the negative consequences associated with using an oil stove, which outweighed its potential benefits.

- Negative Effects of an Oil Stove:
- Leaked oil: One of the main reasons for not taking the oil stove was its tendency to leak oil. This could lead to oil spills and create a messy and unsafe environment. Oil spills can be difficult to clean up and may cause stains or damage to the surrounding area.
- Environmental impact: The leaking oil from the stove would not only create a mess but also have a negative impact on the atmosphere. Oil can release harmful fumes when burned, contributing to air pollution and potentially causing respiratory problems for the friends and others nearby.
- Fire hazard: Another concern with an oil stove is the risk of fire. If the stove is not properly maintained or if there are any leaks or malfunctions, it could lead to a fire hazard, endangering the safety of the friends and their surroundings.

- Weight and Cost considerations:
- While weight and cost were not mentioned as reasons for not taking the oil stove, they can be additional factors to consider. Oil stoves tend to be heavy due to the materials used and the oil reservoir. Carrying a heavy stove could be impractical, especially if the friends were planning to travel long distances or had limited means of transportation.
- Cost can also be a factor as oil stoves may be more expensive to purchase compared to other types of stoves or cooking equipment. If the friends were on a tight budget, they might have opted for a more affordable option that did not have the negative consequences associated with an oil stove.

In conclusion, the friends decided not to take an oil stove with them due to its tendency to leak oil and spoil the atmosphere. The potential negative effects on the environment, safety, and overall convenience outweighed any benefits it may have provided.