The correct option is Steatite.
Explanation
The seals of the Indus Valley Civilization are among the most distinctive artifacts of the period, serving primarily commercial and symbolic functions. While various materials were utilized, a specific soft stone was predominantly chosen for its ease of carving.
- Steatite is Correct: The standard Harappan seal was made of steatite (soapstone). Steatite is a soft metamorphic rock that allowed artisans to carve intricate motifs and scripts with precision. After carving, the seals were typically fired to harden the surface and often coated with an alkali wash to create a white, glazed appearance.
- Basalt is Incorrect: Basalt is a hard volcanic rock. While stone masonry existed, basalt was not the standard material for the delicate and small-scale carving required for seals.
- Terracotta is Incorrect: Terracotta (baked clay) was widely used for pottery, toys, and figurines. Although some terracotta seals have been found, they were less common and lacked the durability and fine detail associated with the standard steatite seals.
- Limestone is Incorrect: Limestone was used for larger sculptures, such as the famous "Bearded Priest" bust found at Mohenjo-Daro, but it was not the primary material for the mass-produced seals.
Key Takeaway: The vast majority of Harappan seals were crafted from steatite due to its softness for carving and ability to harden upon firing.