The correct answer is option D: a terracotta figure.

The Indus Valley Civilization, also known as the Harappan Civilization, existed around 3300-1300 BCE in what is now modern-day Pakistan and northwest India. One of the most significant features of this ancient civilization was the worship of a mother goddess, often depicted in various forms. Among these figures, the terracotta figure holds the most importance for several reasons.

1. Significance of the Mother Goddess:
- The mother goddess was a central figure in the religious beliefs of the Indus Valley Civilization. She was believed to be a powerful deity associated with fertility, creation, and the nurturing aspect of life.
- Her worship was widespread, indicating the importance and reverence given to her in the society of that time.
- The mother goddess represented the life-giving force and the source of abundance and prosperity, making her an integral part of the community's religious and cultural practices.

2. Terracotta Figures:
- Terracotta figures were commonly found in the Indus Valley Civilization, indicating their importance and popularity as religious artifacts.
- These figures were made from baked clay and were often small in size, suggesting that they were used in personal or household rituals and worship.
- The terracotta figures depicting the mother goddess were typically portrayed with exaggerated features, such as a prominent headdress or a swollen abdomen, emphasizing the fertility aspect of the goddess.

3. Cultural and Religious Significance:
- The terracotta figure of the mother goddess represented the connection between the divine and the human realm. It served as a physical representation of the deity, allowing worshippers to establish a personal connection and seek blessings.
- The figure would have been used in rituals and ceremonies, symbolizing the presence of the mother goddess and inviting her divine intervention in matters of fertility, childbirth, and protection.
- The terracotta figures also played a role in ensuring the well-being of the community by fostering a sense of unity and collective identity through shared religious practices.

In conclusion, the terracotta figure representing the mother goddess holds the utmost importance among the Indus figures. It symbolizes the worship of the mother goddess, her role in fertility and abundance, and the cultural and religious significance she held in the Indus Valley Civilization.