Fibrous proteins are insoluble in

Water

The correct answer is (1) water.

Fibrous proteins are insoluble in water because they are made up of long, non-polar chains of amino acids. These chains are not attracted to water molecules, so they do not dissolve in water.

The amino acids in fibrous proteins are typically hydrophobic, meaning that they have a strong aversion to water. This is because the amino acids have long, non-polar side chains that are not attracted to the polar water molecules.

When fibrous proteins are placed in water, the water molecules try to surround the protein chains. However, the hydrophobic side chains of the amino acids repel the water molecules, preventing them from forming hydrogen bonds with the protein chains. This causes the protein chains to clump together and become insoluble.

There are a few exceptions to this rule. Some fibrous proteins, such as silk, are soluble in water. However, these proteins have special properties that allow them to interact with water molecules. For example, silk proteins have an amino acid called glycine, which has a small, polar side chain that is attracted to water molecules. This allows the silk proteins to form hydrogen bonds with water molecules, making them soluble.

In general, however, fibrous proteins are insoluble in water. This is because the amino acids in these proteins are hydrophobic, and they have long, non-polar side chains that are not attracted to water molecules.