Family and its respective members:

The correct combination of family and its respective members is option 'B', which is Solanaceae - Withania somnifera, Petunia.

Explanation:

Family is a higher taxonomic rank in the classification of organisms. It is a group of related plants or animals that share common characteristics and are classified under the same category. Each family consists of multiple members or species that belong to it. Let's take a closer look at each option and identify the correct combination:

a) Fabaceae - Colchicum autumnale, Trifolium alexandrinum:
- Fabaceae, also known as the legume or pea family, includes plants with characteristic pea-like flowers. Colchicum autumnale, also known as autumn crocus, belongs to the family Colchicaceae and not Fabaceae. Trifolium alexandrinum, also known as Egyptian clover, belongs to the Fabaceae family. Hence, this combination is incorrect.

b) Solanaceae - Withania somnifera, Petunia:
- Solanaceae, also known as the nightshade family, includes plants with characteristic bell-shaped flowers. Withania somnifera, commonly known as ashwagandha, and Petunia, commonly known as petunia, both belong to the Solanaceae family. Therefore, this combination is correct.

c) Liliaceae - Sesbania, Asparagus:
- Liliaceae, also known as the lily family, includes plants with characteristic floral structures. Sesbania, also known as sesban, belongs to the Fabaceae family and not Liliaceae. Asparagus, commonly known as asparagus, belongs to the Asparagaceae family and not Liliaceae. Thus, this combination is incorrect.

d) Asteraceae - Sonchus asper, Nicotiana tabacum:
- Asteraceae, also known as the aster or sunflower family, includes plants with characteristic composite flowers. Sonchus asper, also known as prickly sowthistle, belongs to the Asteraceae family. Nicotiana tabacum, commonly known as tobacco, belongs to the Solanaceae family and not Asteraceae. Therefore, this combination is incorrect.

In conclusion, the correct combination of family and its respective members is option 'B' - Solanaceae, which includes Withania somnifera (ashwagandha) and Petunia.