The correct answer is option 'B', a mustard field.

In the story "The Lost Child" by Mulk Raj Anand, the author vividly describes the bustling fairground where the child gets separated from his parents. As the child wanders off, he becomes captivated by the various attractions and starts following them. One of the attractions that caught the child's attention was a mustard field.

A mustard field is compared to melting gold in the story. Here's an explanation of why this comparison is made:

1. Importance of mustard fields:
- Mustard fields hold great significance in rural India as they are a major source of income for farmers.
- They are known for their vibrant yellow flowers and lush green leaves, creating a visually appealing landscape.
- Mustard is also an essential ingredient in Indian cuisine, thus making mustard fields an integral part of the cultural fabric.

2. Visual imagery:
- The comparison of a mustard field to melting gold is used to evoke a sense of beauty and allure in the reader's mind.
- The yellow flowers of the mustard field resemble the bright, shimmering color of melted gold, creating a visual parallel.
- Just as melting gold captures one's attention and is considered precious, the mustard field captivates the child's attention and becomes a significant attraction for him.

3. Symbolism:
- The comparison also symbolizes the child's fascination and desire for the mustard field.
- The child is drawn to the mustard field because of its radiant beauty, much like how gold is valued for its brilliance and allure.
- It represents the child's innocent and pure perception of the world, where even simple natural elements like a mustard field can hold tremendous enchantment.

4. Contrast to the crowd:
- The comparison of the mustard field to melting gold also serves to contrast it with the crowd at the fair.
- While the crowd is described as noisy, colorful, and chaotic, the mustard field stands out as a serene and captivating sight.
- It provides a momentary respite for the child from the overwhelming stimulation of the crowd, allowing him to momentarily forget his lost state.

In conclusion, the mustard field in the story "The Lost Child" is compared to melting gold to highlight its beauty, captivation, and contrast to the chaotic fairground. The comparison serves to enhance the reader's understanding of the child's fascination and the impact of the mustard field on his journey.