Introduction:
The lifespan of an underground cable is an important factor to consider when planning and implementing underground cable systems. The correct answer to the given question is option 'C' - 25 years. In this response, we will discuss the reasons behind this answer and explain why the life of an underground cable is generally considered to be 25 years.

Factors Affecting the Life of Underground Cables:
Several factors can influence the lifespan of underground cables. It is essential to consider these factors while determining the expected life of an underground cable. Some of these factors include:

1. Environmental Conditions: The environmental conditions in which the underground cable is installed play a crucial role in its lifespan. Factors like temperature variations, moisture levels, soil conditions, and exposure to chemicals can impact the cable's longevity.

2. Installation Quality: Proper installation techniques and adherence to industry standards are critical for the longevity of underground cables. Poor installation practices can lead to cable damage or degradation, reducing its lifespan.

3. Cable Design and Material: The design and composition of the cable also affect its lifespan. High-quality materials and robust construction techniques can enhance the cable's durability and longevity.

Reasons behind the 25-Year Lifespan:
The 25-year lifespan for underground cables is based on several considerations and industry practices. Here are some reasons why this duration is commonly accepted:

1. Technical Life: The technical life of an underground cable refers to the period during which it can function efficiently without significant degradation. Industry standards and research suggest that most cables can operate reliably for around 25 years under normal conditions.

2. Economic Considerations: The lifespan of an underground cable is also influenced by economic factors. Balancing the initial investment in cable installation with the expected lifespan helps determine the economic feasibility of underground cable systems. A 25-year lifespan strikes a balance between cost and longevity.

3. Maintenance and Replacement: Regular maintenance and periodic replacement of cables are necessary to ensure uninterrupted power supply and avoid failures. A 25-year lifespan allows for scheduled maintenance and replacement cycles, reducing the risk of unexpected outages.

Conclusion:
In conclusion, the life of an underground cable is commonly considered to be 25 years. This duration takes into account various factors such as environmental conditions, installation quality, cable design, and economic considerations. Adhering to industry standards and conducting regular maintenance can help maximize the lifespan of underground cables and ensure reliable power distribution.