To determine the type of silk that is soft, lustrous, and elastic, we need to analyze the characteristics of each option provided.
A: Kosa silk
- Kosa silk is a type of silk produced by the Antheraea mylitta moth.
- It is known for its coarse texture and dull appearance.
- Therefore, it is not soft, lustrous, and elastic.
B: Mulberry silk moth silk
- Mulberry silk is produced by the Bombyx mori moth.
- It is the most common and widely-used type of silk.
- Mulberry silk is known for its softness, luster, and elasticity.
- Therefore, it matches the given description.
C: Mooga silk
- Mooga silk is produced by the Antheraea assamensis moth.
- It is known for its golden color and durability.
- However, it is not typically described as soft, lustrous, and elastic.
D: Tassar silk
- Tassar silk is produced by various species of wild silk moths, such as Antheraea mylitta and Antheraea paphia.
- It is known for its textured appearance and natural tan color.
- While it is not as soft and lustrous as mulberry silk, it does possess some elasticity.
Based on the given options, the silk fiber that is soft, lustrous, and elastic is Mulberry silk moth silk (Option B).

Mulberry silk moth silk is the soft, lustrous, and elastic silk fiber among the options given.

Mulberry Silk:
Mulberry silk is produced by the silkworm Bombyx mori, commonly known as the mulberry silk moth. This type of silk is highly valued for its superior quality and luxurious feel.

Softness:
Mulberry silk is renowned for its softness and smooth texture. The silk fibers are extremely fine and delicate, giving the fabric a luxurious and gentle touch. The softness of mulberry silk makes it comfortable to wear against the skin, making it a popular choice for various types of clothing.

Luster:
Mulberry silk is known for its natural sheen and luster. The fibers reflect light in a unique way, giving the fabric a beautiful and shiny appearance. This lustrous quality adds to the attractiveness of mulberry silk garments, making them stand out and look elegant.

Elasticity:
Mulberry silk exhibits excellent elasticity, meaning it can stretch and recover its original shape without getting deformed or losing its integrity. This elasticity makes mulberry silk garments more flexible and comfortable to wear, as they can move and adapt to the body's movements.

Comparison with Other Silk Fibers:
a) Kosa Silk: Kosa silk is produced by the Antheraea mylitta silkworm, commonly found in India. While it is known for its durability and rich texture, it is not as soft, lustrous, or elastic as mulberry silk.
c) Mooga Silk: Mooga silk is produced by the Antheraea assamensis silkworm, found in Assam, India. It is known for its golden-yellow color and durability. However, it is not as soft, lustrous, or elastic as mulberry silk.
d) Tassar Silk: Tassar silk is produced by several species of wild silk moths. It has a coarse texture and is less lustrous compared to mulberry silk. Although it is more robust than mulberry silk, it is not as soft or elastic.

In conclusion, mulberry silk is the preferred choice when it comes to softness, luster, and elasticity among the given options of silk fibers. Its luxurious feel, natural sheen, and ability to stretch and recover make it highly desirable for various applications in the textile industry.