Myrosin is a type of enzyme that is found in the plant parts of the Cruciferae family. It is a characteristic feature of this plant family and is not found in the other options mentioned (Malvaceae, Solanaceae, and Liliaceae).

Explanation:

1. Cruciferae Family:
- The Cruciferae family, also known as the Brassicaceae family, includes various plants such as mustard, cabbage, broccoli, cauliflower, and radish.
- These plants are characterized by their four-petaled flowers and the arrangement of their leaves in a cross-like pattern, hence the name Cruciferae.
- They are known for their economic importance as food crops and for their medicinal properties.

2. Myrosin Enzyme:
- Myrosin is an enzyme found in the plant parts of the Cruciferae family, particularly in the seeds and leaves.
- It is involved in the breakdown of glucosinolates, which are sulfur-containing compounds found in these plants.
- When the plant tissue is damaged, myrosin comes into contact with glucosinolates, leading to the formation of various compounds, including isothiocyanates, nitriles, and thiocyanates.
- These compounds are responsible for the characteristic pungent taste and smell of Cruciferae plants and also contribute to their defensive properties against herbivores and pathogens.

3. Other Plant Families:
- Malvaceae: This family includes plants such as hibiscus, cotton, and okra. They do not contain myrosin.
- Solanaceae: This family includes plants such as tomatoes, potatoes, and peppers. They do not contain myrosin.
- Liliaceae: This family includes plants such as lilies, onions, and garlic. They do not contain myrosin.

In conclusion, myrosin is a characteristic feature of the Cruciferae family, and it is not found in the other plant families mentioned as options. Cruciferae plants are known for their economic and medicinal importance, and myrosin plays a crucial role in the breakdown of glucosinolates, leading to the formation of various bioactive compounds.