**Explanation:**

The correct answer is option 'B': bask in the sun.

**Basking in the Sun:**
When we say that the beggar spread his upper cloth under the neem tree to bask in the sun, it means that he positioned himself under the tree in such a way that he could receive direct sunlight. This is a common practice among people, especially during winter or cooler days, as the sun provides warmth and comfort. Basking in the sun helps to regulate body temperature and can provide a sense of relaxation.

**Benefits of Basking in the Sun:**
There are several reasons why the beggar might have chosen to bask in the sun under the neem tree:

1. **Warmth**: The sun's rays provide warmth, especially during colder days. By positioning himself under the neem tree, the beggar could benefit from the heat and feel more comfortable.

2. **Vitamin D**: Sunlight is a natural source of vitamin D, which is essential for the body. By exposing himself to the sun, the beggar could potentially increase his vitamin D levels.

3. **Mood Enhancement**: Sunlight has been known to boost mood and improve mental well-being. The warmth of the sun and the natural light can have a positive effect on a person's mood, helping them feel happier and more content.

4. **Relaxation**: Basking in the sun can be a form of relaxation. The beggar might have chosen to position himself under the neem tree to take a break, unwind, and enjoy the warmth of the sun.

**Conclusion:**
In conclusion, the beggar spread his upper cloth under the neem tree to bask in the sun. This choice allowed him to benefit from the warmth, potentially increase his vitamin D levels, improve his mood, and find relaxation. Basking in the sun can provide physical and mental comfort, making it a common practice for people seeking warmth and relaxation.